fix: bill Grok video per second and harden video usage logging

Follow-up fixes for the #3775 audit findings:

- Bill Grok video generation per second of output, matching the xAI rate
  card: parse the request duration (1-15s, upstream default 8s) and compute
  cost as per-second price x duration x count. The built-in rate card values
  were already xAI per-second prices but were previously charged per video,
  undercharging up to 15x with a user-controlled duration.
- Group video_price_* fields are now documented and surfaced as per-second
  rates (USD/s); admin UI labels, placeholders and hints updated accordingly.
- Persist video_count/video_resolution/video_duration_seconds on usage_logs
  (migration 172) so video billing is auditable, and exempt any row with
  video_count > 0 from the image_size check constraint: a video billed via a
  token-mode channel price produces billing_mode='token' with image_count=1
  and no image_size, which the previous constraint rejected, dropping the
  whole billing transaction.
- Only refetch the group in apiKeyWithFreshGroupMediaPricing when the group
  object actually looks like it is missing media pricing fields (both media
  multipliers zero and all prices nil, impossible for a normally loaded
  group), removing a per-usage DB query for groups without overrides.
- Frontend: drop the unused admin.groups.mediaPricing locale block, map
  cleared price inputs to null (create) / -1 (update, cleared via backend
  normalizePrice) instead of sending "" that failed *float64 unmarshalling,
  and align video price placeholders with the text-to-video default model
  (grok-imagine-video 0.05/0.07, 1080p only on 1.5 at 0.25).

frontend/src/i18n/locales/en/admin/overview.ts

@@ -841,26 +841,15 @@ export default {
         finalPricePreview: 'Final per-image price preview',
         notConfigured: 'Not configured'
       },
-      mediaPricing: {
-        title: 'Image / Video Generation Pricing',
-        description:
-          'Configure Grok image and video generation access plus base media prices. Leave empty to use default prices.',
-        allowImageGeneration: 'Allow image and video generation for this group',
-        independentMultiplier: 'Use independent media multiplier',
-        imageMultiplier: 'Media multiplier',
-        modeHint:
-          'By default, Grok media billing uses media price × current effective group multiplier. Independent mode uses media price × media multiplier. One video generation is billed as one media unit.',
-        finalPricePreview: 'Final per-media-unit price preview',
-        notConfigured: 'Not configured'
-      },
       videoPricing: {
         title: 'Video Generation Pricing',
-        description: 'Configure Grok video generation base prices. Leave empty to use default video prices.',
+        description:
+          'Configure Grok video generation prices in USD per second of output video. Leave empty to use the default per-second rates (grok-imagine-video: $0.05/s 480p, $0.07/s 720p; video-1.5: $0.08/s 480p, $0.14/s 720p, $0.25/s 1080p).',
         independentMultiplier: 'Use independent video multiplier',
         videoMultiplier: 'Video multiplier',
         modeHint:
-          'By default, video billing uses video price × current effective group multiplier. Independent mode uses video price × video multiplier.',
-        finalPricePreview: 'Final per-video price preview',
+          'Videos are billed per second: per-second price × duration (1-15s, default 8s). By default the current effective group multiplier applies; independent mode uses the video multiplier instead.',
+        finalPricePreview: 'Final per-second price preview',
         notConfigured: 'Not configured'
       },
       peakRate: {

frontend/src/i18n/locales/zh/admin/overview.ts

@@ -919,25 +919,15 @@ export default {
         finalPricePreview: '最终单张价格预览',
         notConfigured: '未配置'
       },
-      mediaPricing: {
-        title: '图片/视频生成计费',
-        description: '配置 Grok 图片和视频生成能力及媒体基础单价，留空则使用默认价格',
-        allowImageGeneration: '允许当前分组生图和视频生成',
-        independentMultiplier: '媒体倍率独立',
-        imageMultiplier: '媒体独立倍率',
-        modeHint:
-          '默认关闭独立倍率时，Grok 媒体费用 = 媒体价格 × 当前分组有效倍率；开启独立倍率后，Grok 媒体费用 = 媒体价格 × 媒体独立倍率。一次视频生成按 1 个媒体单位计费。',
-        finalPricePreview: '最终单次媒体价格预览',
-        notConfigured: '未配置'
-      },
       videoPricing: {
         title: '视频生成计费',
-        description: '配置 Grok 视频生成基础单价，留空则使用默认视频价格',
+        description:
+          '配置 Grok 视频生成的每秒单价（USD/秒），留空则使用默认每秒价（grok-imagine-video：480p $0.05/s、720p $0.07/s；video-1.5：480p $0.08/s、720p $0.14/s、1080p $0.25/s）',
         independentMultiplier: '视频倍率独立',
         videoMultiplier: '视频独立倍率',
         modeHint:
-          '默认关闭独立倍率时，视频费用 = 视频价格 × 当前分组有效倍率；开启独立倍率后，视频费用 = 视频价格 × 视频独立倍率。',
-        finalPricePreview: '最终单次视频价格预览',
+          '视频按秒计费：费用 = 每秒价格 × 时长（1-15 秒，未指定默认 8 秒）。默认叠加当前分组有效倍率；开启独立倍率后改用视频独立倍率。',
+        finalPricePreview: '最终每秒价格预览',
         notConfigured: '未配置'
       },
       peakRate: {

frontend/src/views/admin/GroupsView.vue

@@ -1010,7 +1010,7 @@
           </div>
           <div class="grid grid-cols-3 gap-3">
             <div>
-              <label class="input-label">480p ($)</label>
+              <label class="input-label">480p ($/s)</label>
               <input
                 v-model.number="createForm.video_price_480p"
                 type="number"
@@ -1021,7 +1021,7 @@
               />
             </div>
             <div>
-              <label class="input-label">720p ($)</label>
+              <label class="input-label">720p ($/s)</label>
               <input
                 v-model.number="createForm.video_price_720p"
                 type="number"
@@ -1032,7 +1032,7 @@
               />
             </div>
             <div>
-              <label class="input-label">1080p ($)</label>
+              <label class="input-label">1080p ($/s)</label>
               <input
                 v-model.number="createForm.video_price_1080p"
                 type="number"
@@ -2489,7 +2489,7 @@
           </div>
           <div class="grid grid-cols-3 gap-3">
             <div>
-              <label class="input-label">480p ($)</label>
+              <label class="input-label">480p ($/s)</label>
               <input
                 v-model.number="editForm.video_price_480p"
                 type="number"
@@ -2500,7 +2500,7 @@
               />
             </div>
             <div>
-              <label class="input-label">720p ($)</label>
+              <label class="input-label">720p ($/s)</label>
               <input
                 v-model.number="editForm.video_price_720p"
                 type="number"
@@ -2511,7 +2511,7 @@
               />
             </div>
             <div>
-              <label class="input-label">1080p ($)</label>
+              <label class="input-label">1080p ($/s)</label>
               <input
                 v-model.number="editForm.video_price_1080p"
                 type="number"
@@ -4718,6 +4718,14 @@ const handleCreateGroup = async () => {
     requestData.video_rate_multiplier = normalizeRateMultiplier(
       requestData.video_rate_multiplier,
     );
+    // 媒体价格输入清空时 v-model.number 产生 ""，直接提交会被后端 *float64 反序列化拒绝（400），
+    // 创建时按"未配置"（null）处理。
+    requestData.image_price_1k = emptyToNull(requestData.image_price_1k);
+    requestData.image_price_2k = emptyToNull(requestData.image_price_2k);
+    requestData.image_price_4k = emptyToNull(requestData.image_price_4k);
+    requestData.video_price_480p = emptyToNull(requestData.video_price_480p);
+    requestData.video_price_720p = emptyToNull(requestData.video_price_720p);
+    requestData.video_price_1080p = emptyToNull(requestData.video_price_1080p);
     requestData.peak_rate_enabled = createForm.peak_rate_enabled;
     requestData.peak_start = createForm.peak_start;
     requestData.peak_end = createForm.peak_end;
@@ -4896,6 +4904,16 @@ const handleUpdateGroup = async () => {
     payload.video_rate_multiplier = normalizeRateMultiplier(
       payload.video_rate_multiplier,
     );
+    // 媒体价格输入清空时 v-model.number 产生 ""，直接提交会被后端 *float64 反序列化拒绝（400）。
+    // 更新语义中 null 表示"不修改"，因此清空后的字段发送 -1：后端 normalizePrice 将负价归一为
+    // NULL，从而真正清除已配置的价格。
+    const emptyPriceToClear = (v: any) => (v === "" || v === null ? -1 : v);
+    payload.image_price_1k = emptyPriceToClear(payload.image_price_1k);
+    payload.image_price_2k = emptyPriceToClear(payload.image_price_2k);
+    payload.image_price_4k = emptyPriceToClear(payload.image_price_4k);
+    payload.video_price_480p = emptyPriceToClear(payload.video_price_480p);
+    payload.video_price_720p = emptyPriceToClear(payload.video_price_720p);
+    payload.video_price_1080p = emptyPriceToClear(payload.video_price_1080p);
     payload.peak_rate_enabled = editForm.peak_rate_enabled;
     payload.peak_start = editForm.peak_start;
     payload.peak_end = editForm.peak_end;

frontend/src/views/admin/__tests__/groupsImagePricing.spec.ts

@@ -37,8 +37,10 @@ describe("groups image pricing platform support", () => {
   it("uses Grok media defaults instead of generic image fallback placeholders", () => {
     expect(getImagePricePlaceholder("grok", "image_price_1k")).toBe("0.02");
     expect(getImagePricePlaceholder("grok", "image_price_2k")).toBe("0.02");
-    expect(getVideoPricePlaceholder("grok", "video_price_480p")).toBe("0.08");
-    expect(getVideoPricePlaceholder("grok", "video_price_720p")).toBe("0.14");
+    // 视频 placeholder 为每秒单价：480p/720p 取 grok-imagine-video 官方每秒价，
+    // 1080p 仅 video-1.5 支持、取 1.5 每秒价。
+    expect(getVideoPricePlaceholder("grok", "video_price_480p")).toBe("0.05");
+    expect(getVideoPricePlaceholder("grok", "video_price_720p")).toBe("0.07");
     expect(getVideoPricePlaceholder("grok", "video_price_1080p")).toBe("0.25");
   });
 

frontend/src/views/admin/groupsImagePricing.ts

@@ -39,13 +39,15 @@ const defaultImagePricePlaceholders: Record<
   },
 };
 
+// 视频价为每秒单价（USD/s）。480p/720p 取 grok-imagine-video（文生视频实际走该模型）的
+// 官方每秒价；1080p 仅 grok-imagine-video-1.5 图生视频支持，取 1.5 的每秒价。
 const defaultVideoPricePlaceholders: Record<
   string,
   Record<VideoPricingTierKey, string>
 > = {
   grok: {
-    video_price_480p: "0.08",
-    video_price_720p: "0.14",
+    video_price_480p: "0.05",
+    video_price_720p: "0.07",
     video_price_1080p: "0.25",
   },
 };