fix(worker): 将 Gemini 同步请求交由异步队列执行

将非流式 Gemini generateContent 改为 River Worker 执行，API 通过批量状态查询等待完成，避免高并发同步请求耗尽 API 数据库连接池。\n\n生成图片在持久化时保存带哈希的资产引用，响应恢复时下载并校验后重建 Base64，数据库不保存媒体原文。请求体物化增加前置内存门禁，并扩展双 API/Worker PostgreSQL 压力测试。\n\n验证：go test ./... -count=1；go vet ./...；pnpm openapi；迁移安全检查；64 与 256 请求双角色异步 Gemini 压力测试。

apps/api/internal/runner/task_completion_test.go

@@ -0,0 +1,50 @@
+package runner
+
+import (
+	"testing"
+	"time"
+)
+
+func TestTaskCompletionWaitersAreBatchedAndSignaled(t *testing.T) {
+	service := &Service{
+		taskCompletionWaiters:  map[string]map[chan struct{}]struct{}{},
+		taskCompletionPollWake: make(chan struct{}, 1),
+	}
+	first, unregisterFirst := service.registerTaskCompletionWaiter("task-1")
+	defer unregisterFirst()
+	second, unregisterSecond := service.registerTaskCompletionWaiter("task-2")
+
+	taskIDs := service.taskCompletionWaiterIDs(10)
+	if len(taskIDs) != 2 {
+		t.Fatalf("batched task IDs=%v", taskIDs)
+	}
+	service.signalTaskCompletion("task-1")
+	select {
+	case <-first:
+	case <-time.After(time.Second):
+		t.Fatal("task completion waiter was not signaled")
+	}
+	select {
+	case <-second:
+		t.Fatal("unrelated task waiter was signaled")
+	default:
+	}
+
+	unregisterSecond()
+	if got := service.taskCompletionWaiterIDs(10); len(got) != 1 || got[0] != "task-1" {
+		t.Fatalf("waiter unregister left unexpected IDs=%v", got)
+	}
+}
+
+func TestTerminalTaskStatus(t *testing.T) {
+	for _, status := range []string{"succeeded", "failed", "cancelled", "manual_review"} {
+		if !terminalTaskStatus(status) {
+			t.Fatalf("status %q should be terminal", status)
+		}
+	}
+	for _, status := range []string{"queued", "running", "pending"} {
+		if terminalTaskStatus(status) {
+			t.Fatalf("status %q should not be terminal", status)
+		}
+	}
+}