fix: downgrade @fastify/multipart to ^8 and @fastify/static to ^7 (Fastify 4 compat)

v9 of both plugins requires Fastify 5.x — we're on 4.x

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/package.json

@@ -13,9 +13,9 @@
     "@fastify/cors": "^9.0.1",
     "@fastify/helmet": "^11.1.1",
     "@fastify/jwt": "^8.0.1",
-    "@fastify/multipart": "^9.4.0",
+    "@fastify/multipart": "^8.3.0",
     "@fastify/rate-limit": "^9.1.0",
-    "@fastify/static": "^9.0.0",
+    "@fastify/static": "^7.0.4",
     "@fastify/websocket": "^8.3.1",
     "bcryptjs": "^2.4.3",
     "fastify": "^4.28.1",