feat(channel-monitor-v2): 渠道监控展示首次聚合进度

在用户端监控页显示 bootstrap 进度横幅与百分比，完成后自动消失；
bootstrap 期间加快轮询，空态文案与进度联动。

frontend/src/api/channelMonitorV2.ts

@@ -76,6 +76,14 @@ export interface MonitorHealth {
   }
 }
 
+/** First-upgrade historical fill for 90m/24h/7d/30d; omitted when complete. */
+export interface MonitorBootstrap {
+  active: boolean
+  progress_percent: number
+  covered_from?: string
+  target_start?: string
+}
+
 export interface MonitorCoverage {
   requested_start: string
   coverage_start: string
@@ -84,6 +92,8 @@ export interface MonitorCoverage {
   aggregation_lag_seconds: number
   coverage_complete: boolean
   bucket_seconds: number
+  /** Present while initial aggregation has not covered the 30d product window. */
+  bootstrap?: MonitorBootstrap | null
 }
 
 export interface MonitorConfig {