feat: TenantSource 抽象——File源(测试/档案)与DB源(生产)，registry 数据源可切换+reload upsert

Co-Authored-By: Claude Code <noreply@anthropic.com>

app/engine/routes/public.py

@@ -81,7 +81,10 @@ async def avatar(request: Request):
     # 任意文件。显式拒绝分隔符与目录点,命中即回退静态兜底。
     if name in {".", ".."} or "/" in name or "\\" in name:
         name = ""
-    candidate = runtime.seed_dir / "assets" / name if name else None
+    # seed_dir=None = DB 源租户(Task 4;资产目录尚无 DB 侧存储)——直接
+    # 落静态兜底,不让 None 进路径拼接炸 500
+    candidate = (runtime.seed_dir / "assets" / name
+                 if name and runtime.seed_dir is not None else None)
     if candidate is not None and candidate.is_file():
         return FileResponse(
             candidate, headers={"Access-Control-Allow-Origin": "*"})