Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

models/dbfs/dbfile.go

@@ -339,7 +339,7 @@ func findFileMetaByID(ctx context.Context, metaID int64) (*dbfsMeta, error) {
 	} else if ok {
 		return &fileMeta, nil
 	}
-	return nil, nil
+	return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 }
 
 func buildPath(path string) string {