feat: improve scheduler recurrence, DB config, and docs

- Broaden scheduler query window to next N days for proper recurring event expansion (scheduler.py)
- Update DB connection logic for consistent .env loading and fallback (database.py)
- Harden timezone handling and logging in scheduler and DB utils
- Stop auto-deactivating recurring events before recurrence_end (API/events)
- Update documentation to reflect new scheduler, API, and logging behavior

server/routes/groups.py

@@ -16,11 +16,15 @@ groups_bp = Blueprint("groups", __name__, url_prefix="/api/groups")
 
 
 def get_grace_period():
-    """Wählt die Grace-Periode abhängig von ENV."""
+    """Wählt die Grace-Periode abhängig von ENV.
+    
+    Clients send heartbeats every ~65s. Grace period allows 2 missed
+    heartbeats plus safety margin before marking offline.
+    """
     env = os.environ.get("ENV", "production").lower()
     if env == "development" or env == "dev":
-        return int(os.environ.get("HEARTBEAT_GRACE_PERIOD_DEV", "15"))
-    return int(os.environ.get("HEARTBEAT_GRACE_PERIOD_PROD", "180"))
+        return int(os.environ.get("HEARTBEAT_GRACE_PERIOD_DEV", "180"))
+    return int(os.environ.get("HEARTBEAT_GRACE_PERIOD_PROD", "170"))
 
 
 def is_client_alive(last_alive, is_active):