Add minimum total connection threshold for active ratio alerts

Introduced `ALERT_ACTIVE_CONNECTION_RATIO_MIN_TOTAL_CONNECTIONS` to reduce false positives by requiring a minimum number of total connections before evaluating the active connection ratio. Updated the logic and description in relevant files for clarity and configurability.

.env.example

@@ -42,6 +42,9 @@ ENCRYPTION_KEY=REPLACE_WITH_FERNET_KEY
 CORS_ORIGINS=http://localhost:5173,http://localhost:8080
 # Target polling interval in seconds.
 POLL_INTERVAL_SECONDS=30
+# Active Connection Ratio alert is only evaluated when total sessions
+# are at least this number (reduces false positives on low-traffic DBs).
+ALERT_ACTIVE_CONNECTION_RATIO_MIN_TOTAL_CONNECTIONS=5
 # Initial admin bootstrap user (created on first startup if not present).
 INIT_ADMIN_EMAIL=admin@example.com
 INIT_ADMIN_PASSWORD=ChangeMe123!