feat: webhook notification channel with UI and ingest dispatch

Add webhook config (DB/env), JSON POST on events/problems, settings API,
Settings UI section, and notify_dispatch for multi-channel ingest.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/config.py

@@ -50,7 +50,13 @@ class Settings(BaseSettings):
     telegram_chat_id: str = ""
     telegram_min_severity: str = "high"
 
-    # Порог «живости» агента: agent.heartbeat раз в ~12 ч (ssh-monitor)
+    webhook_enabled: bool = False
+    webhook_url: str = ""
+    webhook_secret_header: str = ""
+    webhook_secret: str = ""
+    webhook_min_severity: str = "high"
+
+    # Порог «живости» агента
     sac_heartbeat_stale_minutes: int = 780
 
     # Окно корреляции Problems: host + type + rule в одном open Problem