Add database overview feature with metrics and UI enhancements

This commit introduces a detailed database overview endpoint and service, providing key metrics such as replication status, database sizes, and performance indicators. On the frontend, a new UI section displays these metrics along with improved forms and troubleshooting tips. Enhancements improve user experience by adding informative tooltips and formatting for byte and time values.

.env.example

@@ -17,6 +17,7 @@ JWT_ACCESS_TOKEN_MINUTES=15
 JWT_REFRESH_TOKEN_MINUTES=10080
 # Generate with: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
 ENCRYPTION_KEY=REPLACE_WITH_FERNET_KEY
+# Dev: set to * to allow all origins (credentials disabled automatically)
 CORS_ORIGINS=http://localhost:5173,http://localhost:8080
 POLL_INTERVAL_SECONDS=30
 INIT_ADMIN_EMAIL=admin@example.com