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/api-tests/run.ts

@@ -77,7 +77,16 @@ async function setupDatabase() {
 }
 
 // --- Start backend ---
+async function killPort(port: number) {
+  try {
+    const { execSync } = await import('child_process')
+    execSync(`lsof -ti:${port} | xargs kill -9 2>/dev/null || true`, { stdio: 'pipe' })
+    await new Promise((r) => setTimeout(r, 1000))
+  } catch {}
+}
+
 async function startBackend(): Promise<Subprocess> {
+  await killPort(TEST_PORT)
   const proc = spawn({
     cmd: ['bun', 'run', 'src/main.ts'],
     cwd: new URL('..', import.meta.url).pathname,
@@ -90,6 +99,7 @@ async function startBackend(): Promise<Subprocess> {
       HOST: '0.0.0.0',
       NODE_ENV: 'development',
       LOG_LEVEL: 'error',
+      STORAGE_LOCAL_PATH: '/tmp/forte-test-files',
     },
     stdout: 'pipe',
     stderr: 'pipe',