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/api/v1/events.py

@@ -17,7 +17,7 @@ from app.schemas.list_models import EventDetail, EventListResponse, EventSummary
 from app.services.ingest import ingest_event
 from app.services.problems import maybe_create_problem
 from app.services.schema_validate import validate_event_payload
-from app.services.telegram_notify import notify_event, notify_problem
+from app.services.notify_dispatch import notify_event, notify_problem
 
 router = APIRouter(prefix="/events", tags=["events"])
 logger = logging.getLogger("sac.ingest")