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.

backend/app/core/config.py

@@ -25,6 +25,7 @@ class Settings(BaseSettings):
     encryption_key: str
     cors_origins: str = "http://localhost:5173"
     poll_interval_seconds: int = 30
+    alert_active_connection_ratio_min_total_connections: int = 5
     init_admin_email: str = "admin@example.com"
     init_admin_password: str = "ChangeMe123!"