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>

internal/server/server_test.go

@@ -132,7 +132,7 @@ func newTestPhotos(t *testing.T) *recipes.PhotoCache {
 // configured, over a migrated temp database and a random secret box.
 func newTestSearch(t *testing.T) *recipes.SearchSettingsService {
 	t.Helper()
-	search, err := recipes.NewSearchSettingsService(newTestStore(t), newTestBox(t))
+	search, err := recipes.NewSearchSettingsService(newTestStore(t), newTestBox(t), "")
 	if err != nil {
 		t.Fatal(err)
 	}