Fix tracker pattern review issues

- Fix TotalCount resolver to dispatch by parent type instead
  of always using the uncategorised banner counter
- Sync MCP tracker_type enum with canonical TrackerType values
- Add validation for UpdateTrackerPatternRequest
- Validate tracker_type on CreateTrackerPatternRequest
- Set LastMatchedAt when creating pattern from detection
- Use COALESCE for SOURCE cursor pagination with NULLs
- Make source nullable in CLI tracker-pattern list

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/coredata/tracker_pattern_order_field.go

@@ -37,7 +37,7 @@ func (p TrackerPatternOrderField) Column() string {
 	case TrackerPatternOrderFieldUpdatedAt:
 		return "updated_at"
 	case TrackerPatternOrderFieldSource:
-		return "source"
+		return "COALESCE(source, '')"
 	}
 	panic(fmt.Sprintf("unsupported order by: %s", p))
 }