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/migrations.py

@@ -204,6 +204,14 @@ async def migrate_schema(engine: AsyncEngine) -> None:
                     )
                     logger.info("Added %s column to tracking_config table", col_name)
 
+        # Add Generic Webhook tracking flag to tracking_config if missing
+        if await _has_table(conn, "tracking_config"):
+            if not await _has_column(conn, "tracking_config", "track_webhook_received"):
+                await conn.execute(
+                    text("ALTER TABLE tracking_config ADD COLUMN track_webhook_received INTEGER DEFAULT 1")
+                )
+                logger.info("Added track_webhook_received column to tracking_config table")
+
         # Drop legacy template content columns from template_config
         # (template content moved to template_slot child rows)
         if await _has_table(conn, "template_config"):