fix(upload): restrict cross-user private file access (UPLOAD-1)

- Add access_mode column to w_uploads table (0 = private, 1 = public) and initialize data in a single migration script
- Enforce strict ownership check for private files during download
- Allow public files to follow whitelisted public-access rules
- Default access_mode to public for avatars and private for generic uploads
- Update frontend service to support optional accessMode parameter

docs/swagger.yaml

@@ -420,6 +420,8 @@ definitions:
     type: object
   model.Upload:
     properties:
+      access_mode:
+        type: integer
       created_at:
         type: string
       extension: