chore: enforce gen transaction rule

backend/llm.txt

@@ -11,6 +11,7 @@ This file condenses `backend/docs/dev/http_api.md` + `backend/docs/dev/model.md`
 - DO keep controller methods thin: parse/bind → call `services.*` → return result/error.
 - DO regenerate code after changes (routes/docs/models).
 - MUST: in `backend/app/services`, prefer the generated GORM-Gen DAO (`backend/database/models/*`) for DB access; treat raw `*gorm.DB` usage as a last resort.
+- MUST: service-layer transactions MUST use `models.Q.Transaction(func(tx *models.Query) error { ... })`; DO NOT use raw `*_db.Transaction(...)` / `db.Transaction(...)` in services unless Gen cannot express the required operation.
 - MUST: after adding/removing/renaming any files under `backend/app/services/`, run `atomctl gen service --path ./app/services` to regenerate `backend/app/services/services.gen.go`; DO NOT edit `services.gen.go` manually.
 - DO add `// @provider` above every controller/service `struct` declaration.
 - DO keep HTTP middlewares in `backend/app/middlewares/` only.
@@ -175,7 +176,7 @@ Reference: `backend/llm.gorm_gen.txt`.
 - MUST: use Gen transaction wrapper so all queries share the same tx connection:
   - `models.Q.Transaction(func(tx *models.Query) error { ... })`
   - Inside tx, use `tx.<Table>.QueryContext(ctx)` / `tx.<Table>.WithContext(ctx)`
-- DO NOT: use `_db.WithContext(ctx).Transaction(...)` in services unless Gen cannot express a required operation.
+- DO NOT: use `_db.WithContext(ctx).Transaction(...)` / `db.Transaction(...)` in services unless Gen cannot express a required operation.
 
 ### 3.3 Updates