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/services/account.service.ts

@@ -607,8 +607,8 @@ export const MemberIdentifierService = {
         issuingAuthority: input.issuingAuthority,
         issuedDate: input.issuedDate,
         expiresAt: input.expiresAt,
-        imageFrontUrl: input.imageFrontUrl,
-        imageBackUrl: input.imageBackUrl,
+        imageFrontFileId: input.imageFrontFileId,
+        imageBackFileId: input.imageBackFileId,
         notes: input.notes,
         isPrimary: input.isPrimary,
       })