Sign-in becomes optional: KANIDM_URL unset disables auth cleanly

A content-only instance (westra preview) has no review desk and no
Kanidm client; booting no longer demands one. Auth routes answer 503
'sign-in is not configured on this instance'; everything public
renders as usual.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y42TyF8Zu7NGRR2893vNcZ

src/server/oidc.rs

@@ -33,6 +33,13 @@ type OidcClient = CoreClient<
 >;
 
 pub struct Oidc {
+    /// `None` when `KANIDM_URL` is unset: a content-only instance with
+    /// sign-in disabled - auth routes answer 503, everything public
+    /// renders as usual.
+    inner: Option<OidcInner>,
+}
+
+struct OidcInner {
     client: OidcClient,
     http: openidconnect::reqwest::Client,
 }
@@ -45,7 +52,13 @@ const REDIRECT_KEY: &str = "oidc_post_login_redirect";
 impl Oidc {
     /// Discovers the provider and builds the client from environment:
     /// `KANIDM_URL`, `OAUTH2_CLIENT_ID`, `OAUTH2_CLIENT_SECRET`, `PUBLIC_URL`.
+    /// With `KANIDM_URL` unset, sign-in is disabled instead of fatal -
+    /// the shape of a public content instance without a review desk.
     pub async fn from_env() -> anyhow::Result<Self> {
+        if std::env::var("KANIDM_URL").is_err() {
+            tracing::warn!("KANIDM_URL not set - sign-in disabled on this instance");
+            return Ok(Self { inner: None });
+        }
         let kanidm_url = require_env("KANIDM_URL")?;
         let client_id = require_env("OAUTH2_CLIENT_ID")?;
         let client_secret = require_env("OAUTH2_CLIENT_SECRET")?;
@@ -75,7 +88,16 @@ impl Oidc {
         )
         .set_redirect_uri(redirect);
 
-        Ok(Self { client, http })
+        Ok(Self {
+            inner: Some(OidcInner { client, http }),
+        })
+    }
+
+    fn configured(&self) -> Result<&OidcInner, HandlerError> {
+        self.inner.as_ref().ok_or((
+            StatusCode::SERVICE_UNAVAILABLE,
+            "sign-in is not configured on this instance".to_string(),
+        ))
     }
 }
 
@@ -118,10 +140,10 @@ pub async fn login(
         session.insert(REDIRECT_KEY, redirect).await.map_err(internal)?;
     }
 
+    let oidc = state.oidc.configured()?;
     let (pkce_challenge, pkce_verifier) = PkceCodeChallenge::new_random_sha256();
 
-    let (auth_url, csrf_state, nonce) = state
-        .oidc
+    let (auth_url, csrf_state, nonce) = oidc
         .client
         .authorize_url(
             CoreAuthenticationFlow::AuthorizationCode,
@@ -182,7 +204,7 @@ pub async fn callback(
         ));
     }
 
-    let oidc = &state.oidc;
+    let oidc = state.oidc.configured()?;
     let token_response = oidc
         .client
         .exchange_code(AuthorizationCode::new(params.code))