fix: production API URL, k8s single replica, health probes

- Change API_URL default from localhost:4000 to empty string (relative URLs)
- Set VITE_API_URL="" explicitly in Dockerfile frontend build stage
- Reduce k8s replicas to 1 (SQLite + ReadWriteOnce PVC)
- Add liveness and readiness probes targeting /health endpoint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -13,6 +13,7 @@ RUN npm ci
 COPY frontend/ ./
 
 # Build frontend for production
+ENV VITE_API_URL=""
 RUN npm run build
 
 # Production stage