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>

pnpm-lock.yaml

@@ -87,7 +87,7 @@ importers:
         specifier: ^4.2.2
         version: 4.2.2(vite@6.4.1(@types/node@22.19.15)(jiti@2.6.1)(lightningcss@1.32.0)(terser@5.46.1)(tsx@4.21.0))
       better-auth:
-        specifier: ^1.0.0
+        specifier: ^1.5.6
         version: 1.5.6(@opentelemetry/api@1.9.1)(react-dom@19.2.4(react@19.2.4))(react@19.2.4)(vitest@3.2.4(@types/node@22.19.15)(jiti@2.6.1)(jsdom@26.1.0)(lightningcss@1.32.0)(terser@5.46.1)(tsx@4.21.0))
       lucide-react:
         specifier: ^0.577.0