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>

frontend/src/config/api.js

@@ -1,4 +1,4 @@
 // API Configuration
-const API_URL = import.meta.env.VITE_API_URL || 'http://localhost:4000';
+const API_URL = import.meta.env.VITE_API_URL || '';
 
 export default API_URL;