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/database/models.py

@@ -160,6 +160,9 @@ class TrackingConfig(SQLModel, table=True):
     track_ups_replace_battery: bool = Field(default=True)
     track_ups_overload: bool = Field(default=True)
 
+    # Generic Webhook event tracking
+    track_webhook_received: bool = Field(default=True)
+
     # Immich asset display
     track_images: bool = Field(default=True)
     track_videos: bool = Field(default=True)