fix: 4 API bugs found by integration tests

1. tags.owner_id column missing — Migration 0105 adds owner_id to tags table
2. contacts trigger first_name vs firstname — Migration 0105 recreates
   unified_search TSV trigger with correct column names (firstname, surname, etc.)
3. create_webhook() missing is_system_admin param — Add to webhook_service.py
4. Missing GET /api/v1/search endpoint — Add to unified_search/routes.py
   with shared _do_search() helper for GET+POST

Alembic head: 0104 → 0105

app/services/webhook_service.py

@@ -113,6 +113,7 @@ async def create_webhook(
     tenant_id: uuid.UUID,
     user_id: uuid.UUID,
     data: dict[str, Any],
+    is_system_admin: bool = False,
 ) -> Webhook:
     """Create a new webhook subscription."""
     webhook = Webhook(