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>

deploy/.env.example

@@ -68,10 +68,10 @@ MEALQUEST_SESSION_MAX=4320h
 # MEALQUEST_OIDC_CLIENT_SECRET_FILE=/run/secrets/oidc_client_secret
 #   The Authentik provider's client secret.
 # MEALQUEST_SEARXNG_URL=http://searxng:8080
-#   The SearXNG recipe-search sidecar on the compose network. Planned; not
-#   read by the service yet. The app takes its recipe-search provider and
-#   base URL from PATCH /api/v1/settings/recipe-search; see
-#   deploy/RUNBOOK.md "Recipe search (SearXNG)".
+#   The SearXNG recipe-search sidecar on the compose network. The Settings
+#   screen pre-fills it as the SearXNG URL until one is saved; the saved
+#   provider and base URL come from PATCH /api/v1/settings/recipe-search.
+#   See deploy/RUNBOOK.md "Recipe search (SearXNG)".
 
 # --- Planned; not read by the service yet -------------------------------------
 #