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>

README.md

@@ -44,6 +44,9 @@ export MEALQUEST_OIDC_ALLOWED_GROUPS=mealquest-admin
 # X-Forwarded-For/-Proto headers are honoured (production: Traefik). Unset
 # trusts no peer, which is right for local development.
 # export MEALQUEST_TRUSTED_PROXY=10.60.1.33
+# Optional: a SearXNG instance the Settings screen suggests as the recipe
+# search URL until one is saved.
+# export MEALQUEST_SEARXNG_URL=http://localhost:8888
 export MEALQUEST_DEFAULT_TZ=America/New_York
 make dev
 ```
@@ -53,8 +56,9 @@ make dev
 http://localhost:5173. Configuration is environment-only and fails fast:
 every variable above is required except the commented-out session
 lifetimes (when set they must be positive, with MAX at least IDLE),
-trusted proxy list (every entry must be an IP or CIDR) and
-`MEALQUEST_DEFAULT_TZ`, and
+trusted proxy list (every entry must be an IP or CIDR), the SearXNG URL
+(an absolute `http://` or `https://` URL without credentials, a query or a
+fragment) and `MEALQUEST_DEFAULT_TZ`, and
 secrets are read only from files named by `*_FILE` variables. `MEALQUEST_PUBLIC_URL`, `MEALQUEST_OIDC_ISSUER` and the
 endpoints the issuer advertises must be `https://`; plain `http://` is
 accepted only for `localhost`, `127.0.0.1` and `[::1]`.