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/api/template_configs.py

@@ -251,6 +251,8 @@ async def get_template_variables(
             "variables": {**scheduled_vars, "assets": "List of asset dicts (use {% for asset in assets %})"},
             "asset_fields": asset_fields,
         },
+        # --- Generic Webhook slots ---
+        **_webhook_variables(),
         # --- Gitea slots ---
         **_gitea_variables(),
         # --- Planka slots ---
@@ -271,6 +273,23 @@ async def get_template_variables(
     }
 
 
+def _webhook_variables() -> dict:
+    return {
+        "message_webhook_received": {
+            "description": "Incoming webhook event notification",
+            "variables": {
+                "service_name": "Provider instance name",
+                "event_type_raw": "Raw event type from payload (or 'webhook_received')",
+                "collection_name": "Collection extracted from payload via collection_path (or empty)",
+                "source_ip": "IP address of the webhook sender",
+                "raw_payload": "Full JSON payload as dict (use raw_payload.field or raw_payload | tojson)",
+                "timestamp": "When the webhook was received",
+                "target_type": "Target type: 'telegram' or 'webhook'",
+            },
+        },
+    }
+
+
 def _gitea_variables() -> dict:
     common = {
         "sender": "Username who triggered the event",