feat(opencode): add OpenCode platform with Zen and GO account types

Keep the internal platform ID opencode_go. Product surface is OpenCode
with account_mode zen|go, model-aware routing across Chat Completions,
Responses, and Anthropic Messages, and X-OpenCode-Session derived from
prompt_cache_key / metadata.user_id so prompt cache can hit.

Unmatched models fall back to /v1/chat/completions. Related to #6742.

backend/migrations/238_opencode_go_platform.sql

@@ -0,0 +1,62 @@
+-- Add OpenCode as a first-class platform (account types Zen / GO).
+--
+-- 1. user_platform_quotas.platform CHECK
+-- 2. composite_model_routes.target_platform CHECK
+-- 3. channel_monitors / channel_monitor_request_templates provider CHECK
+--
+-- Runs after 237_add_minimax_platform.sql. DROP ... IF EXISTS + 幂等守卫保证可重入；
+-- 新约束是 237 的超集，必须同时保留 MiniMax。
+
+ALTER TABLE user_platform_quotas
+    DROP CONSTRAINT IF EXISTS user_platform_quotas_platform_check;
+
+ALTER TABLE user_platform_quotas
+    ADD CONSTRAINT user_platform_quotas_platform_check
+    CHECK (platform IN ('anthropic', 'openai', 'gemini', 'antigravity', 'grok',
+                        'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'));
+
+ALTER TABLE composite_model_routes
+    DROP CONSTRAINT IF EXISTS composite_model_routes_target_platform_check;
+
+ALTER TABLE composite_model_routes
+    ADD CONSTRAINT composite_model_routes_target_platform_check
+    CHECK (target_platform IN ('anthropic', 'openai', 'gemini', 'antigravity', 'grok',
+                               'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'));
+
+DO $$
+DECLARE
+    monitor_constraint_def TEXT;
+    template_constraint_def TEXT;
+BEGIN
+    SELECT pg_get_constraintdef(c.oid)
+      INTO monitor_constraint_def
+      FROM pg_constraint c
+      JOIN pg_class t ON t.oid = c.conrelid
+     WHERE t.relname = 'channel_monitors'
+       AND c.conname = 'channel_monitors_provider_check';
+
+    IF monitor_constraint_def IS NULL OR position('opencode_go' IN monitor_constraint_def) = 0 THEN
+        ALTER TABLE channel_monitors
+            DROP CONSTRAINT IF EXISTS channel_monitors_provider_check;
+        ALTER TABLE channel_monitors
+            ADD CONSTRAINT channel_monitors_provider_check
+            CHECK (provider IN ('openai', 'anthropic', 'gemini', 'grok',
+                                'antigravity', 'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'));
+    END IF;
+
+    SELECT pg_get_constraintdef(c.oid)
+      INTO template_constraint_def
+      FROM pg_constraint c
+      JOIN pg_class t ON t.oid = c.conrelid
+     WHERE t.relname = 'channel_monitor_request_templates'
+       AND c.conname = 'channel_monitor_request_templates_provider_check';
+
+    IF template_constraint_def IS NULL OR position('opencode_go' IN template_constraint_def) = 0 THEN
+        ALTER TABLE channel_monitor_request_templates
+            DROP CONSTRAINT IF EXISTS channel_monitor_request_templates_provider_check;
+        ALTER TABLE channel_monitor_request_templates
+            ADD CONSTRAINT channel_monitor_request_templates_provider_check
+            CHECK (provider IN ('openai', 'anthropic', 'gemini', 'grok',
+                                'antigravity', 'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'));
+    END IF;
+END $$;

backend/migrations/opencode_go_platform_migration_test.go

@@ -0,0 +1,29 @@
+package migrations
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOpenCodeGoPlatformMigration(t *testing.T) {
+	content, err := FS.ReadFile("238_opencode_go_platform.sql")
+	require.NoError(t, err)
+
+	sql := strings.Join(strings.Fields(string(content)), " ")
+	require.Contains(t, sql, "user_platform_quotas_platform_check")
+	require.Contains(t, sql, "composite_model_routes_target_platform_check")
+	require.Contains(t, sql, "channel_monitors_provider_check")
+	require.Contains(t, sql, "channel_monitor_request_templates_provider_check")
+	require.Contains(t, sql, "'opencode_go'")
+	require.Contains(t, sql, "'minimax'")
+	require.Contains(t, sql, "position('opencode_go' IN monitor_constraint_def) = 0")
+	require.Contains(t, sql, "position('opencode_go' IN template_constraint_def) = 0")
+	require.Contains(t, sql,
+		"CHECK (platform IN ('anthropic', 'openai', 'gemini', 'antigravity', 'grok', 'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'))")
+	require.Contains(t, sql,
+		"CHECK (target_platform IN ('anthropic', 'openai', 'gemini', 'antigravity', 'grok', 'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'))")
+	require.Contains(t, sql,
+		"CHECK (provider IN ('openai', 'anthropic', 'gemini', 'grok', 'antigravity', 'kimi', 'zhipu', 'deepseek', 'minimax', 'opencode_go'))")
+}