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/sample_context.py

@@ -189,4 +189,8 @@ _SAMPLE_CONTEXT = {
     "custom_vars": {"team": "Engineering", "message": "Time for standup!"},
     "team": "Engineering",
     "message": "Time for standup!",
+    # Generic Webhook variables (for webhook provider templates)
+    "raw_payload": {"action": "opened", "issue": {"title": "Bug report", "number": 1}, "sender": {"login": "user1"}},
+    "event_type_raw": "webhook_received",
+    "source_ip": "192.168.1.100",
 }