fix(GRO-563): stabilize OAuth login - upgrade better-auth, fix service worker, add 503 handling

- apps/web: upgrade better-auth from ^1.0.0 to ^1.5.6 (matches API)
- apps/web/vite.config.ts: exclude /api/auth/* from service worker caching
- apps/api/index.ts: return 503 when auth not configured
- apps/api/middleware/auth.ts: return 503 when auth not initialized

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/package.json

@@ -15,7 +15,7 @@
   "dependencies": {
     "@groombook/types": "workspace:*",
     "@tailwindcss/vite": "^4.2.2",
-    "better-auth": "^1.0.0",
+    "better-auth": "^1.5.6",
     "lucide-react": "^0.577.0",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",