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/package.json

@@ -17,6 +17,7 @@
   "dependencies": {
     "@fastify/cors": "^10",
     "@fastify/jwt": "^9",
+    "@fastify/multipart": "^9.4.0",
     "@fastify/rate-limit": "^10.3.0",
     "@forte/shared": "workspace:*",
     "bcrypt": "^6",