feat: generic webhook provider with JSONPath payload extraction

Add a new "webhook" provider type that accepts arbitrary HTTP POST payloads,
extracts template variables via user-defined JSONPath mappings, and dispatches
notifications through the existing pipeline. Supports three auth modes
(HMAC-SHA256, Bearer token, none), bounded JSONPath cache, and 1MB payload limit.

Full stack: core provider + event parser, API endpoint, DB migration,
capabilities, seeds, default templates (EN/RU), frontend descriptor, i18n.

packages/server/src/notify_bridge_server/services/dispatch_helpers.py

@@ -84,6 +84,8 @@ def event_allowed_by_config(event: ServiceEvent, tc: TrackingConfig) -> bool:
         "task_completed": tc.track_task_completed,
         # Scheduler events
         "scheduled_message": tc.track_scheduled_message,
+        # Generic Webhook events
+        "webhook_received": tc.track_webhook_received,
         # NUT (UPS) events
         "ups_online": tc.track_ups_online,
         "ups_on_battery": tc.track_ups_on_battery,