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/plugins/builtins/unified_search/routes.py

@@ -42,13 +42,27 @@ router = APIRouter(prefix="/api/v1/search", tags=["search"])
 
 # ─── Search ───
 
-@router.post("", dependencies=[Depends(require_permission("search:read"))])
-async def search(
-    req: SearchRequest,
+@router.get("", dependencies=[Depends(require_permission("search:read"))])
+async def search_get(
+    q: str = Query(..., min_length=1, max_length=500, description="Search query"),
+    entity_types: str | None = Query(None, description="Comma-separated entity types to search"),
+    limit: int = Query(default=20, ge=1, le=100),
+    offset: int = Query(default=0, ge=0),
     current_user: dict = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
 ) -> SearchResponse:
-    """Perform hybrid search with KI query understanding."""
+    """Perform hybrid search via GET (same as POST but with query params)."""
+    types_list = entity_types.split(",") if entity_types else None
+    req = SearchRequest(query=q, entity_types=types_list, limit=limit, offset=offset)
+    return await _do_search(req, current_user, db)
+
+
+async def _do_search(
+    req: SearchRequest,
+    current_user: dict,
+    db: AsyncSession,
+) -> SearchResponse:
+    """Shared search logic used by both GET and POST endpoints."""
     tenant_id = uuid.UUID(current_user["tenant_id"])
     user_id = uuid.UUID(current_user["user_id"])
     is_system_admin = current_user.get("is_system_admin", False)
@@ -107,6 +121,16 @@ async def search(
     )
 
 
+@router.post("", dependencies=[Depends(require_permission("search:read"))])
+async def search(
+    req: SearchRequest,
+    current_user: dict = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+) -> SearchResponse:
+    """Perform hybrid search with KI query understanding."""
+    return await _do_search(req, current_user, db)
+
+
 # ─── Suggest / Autocomplete ───
 
 @router.get("/suggest", dependencies=[Depends(require_permission("search:read"))])