Pre-fill the SearXNG URL from the deployment default

The service now reads the optional MEALQUEST_SEARXNG_URL, validates it at
startup like any SearXNG URL, and returns it in the recipe search settings
as the read-only default_base_url. base_url stays the saved URL only, and
PATCH is unchanged: nothing is stored until the household saves.

The recipe search card pre-fills the SearXNG URL from default_base_url
while no URL is saved, and an edit now clears the last Test result so a
stale count or error never sits beside "Save your changes to test them."

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/server/main.go

@@ -79,9 +79,9 @@ func run(cfg config.Config, logger *slog.Logger) error {
 	if err != nil {
 		return err
 	}
-	search, err := recipes.NewSearchSettingsService(db, box)
+	search, err := recipes.NewSearchSettingsService(db, box, cfg.SearXNGURL)
 	if err != nil {
-		return err
+		return fmt.Errorf("%s: %w", config.EnvSearXNGURL, err)
 	}
 
 	ai, err := aisettings.New(db, box, cfg.PublicURL.String())