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/config/config_test.go

@@ -422,6 +422,8 @@ func TestLoadValues(t *testing.T) {
 		EnvOIDCAllowedGroups:    " mealquest-admin ,family",
 
 		EnvDefaultTZ: "Europe/London",
+
+		EnvSearXNGURL: "http://searxng:8080",
 	}
 	cfg, err := load(func(k string) (string, bool) { v, ok := env[k]; return v, ok })
 	if err != nil {
@@ -480,6 +482,9 @@ func TestLoadValues(t *testing.T) {
 	if cfg.DefaultTZ != "Europe/London" {
 		t.Errorf("DefaultTZ = %q, want Europe/London", cfg.DefaultTZ)
 	}
+	if cfg.SearXNGURL != "http://searxng:8080" {
+		t.Errorf("SearXNGURL = %q, want http://searxng:8080", cfg.SearXNGURL)
+	}
 }
 
 func TestSecretRedacted(t *testing.T) {