Implement file storage layer with local provider, upload/download API, tests

- StorageProvider interface with LocalProvider (S3 placeholder)
- File table with entity_type/entity_id references, content type, path
- POST /v1/files (multipart upload), GET /v1/files (list by entity),
  GET /v1/files/:id (metadata), GET /v1/files/serve/* (content),
  DELETE /v1/files/:id
- member_identifier drops base64 columns, uses file_id FKs
- File validation: type whitelist, size limits, per-entity max
- Fastify storage plugin injects provider into app
- 6 API tests for upload, list, get, delete, validation
- Test runner kills stale port before starting backend

packages/backend/src/db/migrations/meta/_journal.json

@@ -85,6 +85,13 @@
       "when": 1774710000000,
       "tag": "0011_member_address",
       "breakpoints": true
+    },
+    {
+      "idx": 12,
+      "version": "7",
+      "when": 1774720000000,
+      "tag": "0012_file_storage",
+      "breakpoints": true
     }
   ]
 }