feat(worker): 实现集群限流与自适应负载

保留平台模型 RPM、TPM 和并发策略语义，增加 PostgreSQL 集群级租约、饱和候选重选和多平台自动负载，避免突发任务固定等待首个平台。\n\n新增 Worker 实时负载采样、自适应 active/heavy 容量、心跳与管理端指标，并扩展本地 acceptance runner，覆盖三 Worker、同模型三平台 2/4/6 并发和 48 个带图视频突发任务。\n\n验证：go test ./...、go vet ./...、PostgreSQL 跨 Store 集成测试、gofmt、bash -n、ShellCheck 及本地集群 provider-burst 验收通过；48/48 成功，无越限、重复提交、重复计费、重复回调或终态资源泄漏。

apps/api/internal/store/tasks_runtime.go

@@ -757,7 +757,7 @@ WHERE task_id = $1::uuid
 		}
 		if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, taskID); err != nil {
 			return err
@@ -922,7 +922,7 @@ RETURNING `+gatewayTaskColumns, taskID, message))
 		changed = true
 		if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, taskID); err != nil {
 			return err
@@ -1011,7 +1011,7 @@ WHERE task_id = $1::uuid
 		}
 		if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, taskID); err != nil {
 			return err
@@ -1720,7 +1720,7 @@ ON CONFLICT (task_id, event_type) DO NOTHING`,
 		if input.FinalizeAdmission {
 			if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, input.TaskID); err != nil {
 				return err
@@ -2096,7 +2096,7 @@ ON CONFLICT (task_id, event_type) DO NOTHING`, input.TaskID, string(payloadJSON)
 		if input.FinalizeAdmission {
 			if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, input.TaskID); err != nil {
 				return err