refactor(utils): one PgTextString for the four pgtype.Text unwrappers

handlers textToString, router textToString, router getText, and utils
normText were four copies of the same NULL-to-empty unwrap (99 call
sites). One exported utils.PgTextString replaces them all; the private
normText (comparison normalizer) stays but is now the odd one out.

Harness fix found on the way: setupTestServer built a full wiring
literal for route registration and then RETURNED a second, sparser
literal that never copied AnnotationService — setup.AnnotationService
was always nil on the struct every test receives. Nobody noticed until
cross_library_move_test became the first caller (it previously used
the now-retired plain CreateMediaHighlight query); it now exercises
the real SaveHighlight path through the wired service.

internal/router/search.go

@@ -116,11 +116,3 @@ func handleSearchHTML(c *echo.Context, cfg *Config) error {
 	}
 	return c.HTML(http.StatusOK, buf.String())
 }
-
-// textToString converts pgtype.Text to string
-func textToString(t pgtype.Text) string {
-	if t.Valid {
-		return t.String
-	}
-	return ""
-}