refactor: use relative URLs and centralize API endpoints

- Update Django URLs to serve all APIs under /api/ prefix
- Change frontend to use relative URLs (empty baseURL) instead of environment-specific domains
- Create centralized endpoints.ts for maintainable API URL management
- Update all frontend components to use centralized endpoints
- Remove all VITE_API_BASE_URL and REACT_APP_API_BASE_URL dependencies
- Add helper functions for dynamic endpoints with parameters

This ensures the same Docker image works in both production and sandbox
environments without requiring environment-specific configuration.

Fixes:
- Frontend calling old domain (balancer.live.k8s.phl.io)
- API calls failing after domain migration
- /login and /adminportal pages not working

Closes #431

Author: TineoC

server/balancer_backend/settings.py

@@ -106,13 +106,13 @@
 
 # Build database configuration
 db_config = {
-    "ENGINE": os.environ.get("SQL_ENGINE", "django.db.backends.sqlite3"),
-    "NAME": os.environ.get("SQL_DATABASE", BASE_DIR / "db.sqlite3"),
-    "USER": os.environ.get("SQL_USER", "user"),
-    "PASSWORD": os.environ.get("SQL_PASSWORD", "password"),
+        "ENGINE": os.environ.get("SQL_ENGINE", "django.db.backends.sqlite3"),
+        "NAME": os.environ.get("SQL_DATABASE", BASE_DIR / "db.sqlite3"),
+        "USER": os.environ.get("SQL_USER", "user"),
+        "PASSWORD": os.environ.get("SQL_PASSWORD", "password"),
     "HOST": SQL_HOST,
-    "PORT": os.environ.get("SQL_PORT", "5432"),
-}
+        "PORT": os.environ.get("SQL_PORT", "5432"),
+    }
 
 # Configure SSL/TLS based on connection type
 # CloudNativePG within cluster typically doesn't require SSL