perf(worker): 消除运行态计数跨城热行阻塞

将仅用于候选负载均衡的 runtime_client_states 分配和释放写入改为事务级异步提交，避免同步复制期间持续持有单候选热行锁。\n\n运行态计数仍由本地 WAL 持久化，并在运行时恢复中按 running attempt 重建；任务、账务、租约和状态提交不受影响。新增 PostgreSQL 并发与恢复集成测试。\n\n验证：go vet ./...；go test ./... -count=1；64 并发 PostgreSQL 集成测试；迁移安全检查；gofmt；git diff --check。

apps/api/internal/store/tasks_runtime.go

@@ -2156,7 +2156,19 @@ ON CONFLICT (task_id, seq, callback_url) DO NOTHING`,
 }
 
 func (s *Store) RecordClientAssignment(ctx context.Context, candidate RuntimeModelCandidate) error {
-	_, err := s.pool.Exec(ctx, `
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return err
+	}
+	defer rollbackTransaction(tx)
+	// This counter only influences candidate load balancing and is rebuilt from
+	// active attempts during runtime recovery. Keeping a synchronous-replication
+	// commit open while updating one row per upstream client serializes every
+	// assignment for a hot model across regions.
+	if _, err := tx.Exec(ctx, `SET LOCAL synchronous_commit = off`); err != nil {
+		return err
+	}
+	if _, err := tx.Exec(ctx, `
 INSERT INTO runtime_client_states (
   client_id, platform_id, provider, method_name, queue_key, running_count, last_assigned_at
 )
@@ -2170,16 +2182,31 @@ SET running_count = runtime_client_states.running_count + 1,
 		candidate.Provider,
 		candidate.ModelType,
 		candidate.QueueKey,
-	)
-	return err
+	); err != nil {
+		return err
+	}
+	return tx.Commit(ctx)
 }
 
 func (s *Store) RecordClientRelease(ctx context.Context, clientID string, lastError string) error {
-	_, err := s.pool.Exec(ctx, `
+	tx, err := s.pool.Begin(ctx)
+	if err != nil {
+		return err
+	}
+	defer rollbackTransaction(tx)
+	// runtime_client_states is derived scheduler state; see
+	// RecordClientAssignment for why it must not hold its hot row lock through
+	// a cross-region synchronous replication wait.
+	if _, err := tx.Exec(ctx, `SET LOCAL synchronous_commit = off`); err != nil {
+		return err
+	}
+	if _, err := tx.Exec(ctx, `
 UPDATE runtime_client_states
 SET running_count = GREATEST(running_count - 1, 0),
     last_error = NULLIF($2::text, ''),
     updated_at = now()
-WHERE client_id = $1`, clientID, lastError)
-	return err
+WHERE client_id = $1`, clientID, lastError); err != nil {
+		return err
+	}
+	return tx.Commit(ctx)
 }