feat(acceptance): 增加生产同构媒体压力验收模式

引入动态流量门禁、隔离验收身份与协议级 Gemini/Volces 模拟器，覆盖双站点 API、Worker、PostgreSQL、River、账务、回调和媒体物化链路。

新增 P24/P28/P32 容量阶梯、Worker 强杀恢复、真实小流量 canary、CAS 放量和失败保持 validation 的生产编排；Worker 执行槽、连接池、媒体并发和双站点副本数改为环境配置。

验证：Go 全量测试、真实 PostgreSQL 迁移集成测试、迁移安全检查、OpenAPI 生成、ShellCheck、Kustomize、gofmt 和 git diff --check。

apps/api/internal/store/postgres_pool_metrics.go

@@ -0,0 +1,25 @@
+package store
+
+type PostgresPoolMetricsSnapshot struct {
+	MaxConnections       int32
+	TotalConnections     int32
+	AcquiredConnections  int32
+	IdleConnections      int32
+	EmptyAcquireCount    int64
+	CanceledAcquireCount int64
+}
+
+func (s *Store) PostgresPoolMetrics() PostgresPoolMetricsSnapshot {
+	if s == nil || s.pool == nil {
+		return PostgresPoolMetricsSnapshot{}
+	}
+	statistics := s.pool.Stat()
+	return PostgresPoolMetricsSnapshot{
+		MaxConnections:       statistics.MaxConns(),
+		TotalConnections:     statistics.TotalConns(),
+		AcquiredConnections:  statistics.AcquiredConns(),
+		IdleConnections:      statistics.IdleConns(),
+		EmptyAcquireCount:    statistics.EmptyAcquireCount(),
+		CanceledAcquireCount: statistics.CanceledAcquireCount(),
+	}
+}