feat: harden auth, shutdown, and session bundle (0.35.0)

Three features bundled into one release:

- rate-limit /auth/login, /register, /me/password (token bucket,
  5 req/sec sustained with 10-request burst by default; 429 +
  Retry-After header on hit; tracing::warn! per hit so operators
  see attack patterns; AUTH_RATE_PER_SEC / AUTH_RATE_BURST env knobs)
- handle SIGTERM for graceful container stops (replaces bare
  ctrl_c() with a select over ctrl_c + SignalKind::terminate() so
  docker compose stop runs the daemon shutdown path instead of
  letting Chromium leak past SIGKILL)
- clear session.user on 401 from any API call (setOn401Hook in
  api/client.ts, registered from session.svelte.ts gated on
  $app/environment::browser so the SSR bundle never installs it;
  fixes "logged in but no bookmarks/collections" mid-session
  expiry state)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/api/auth.rs

@@ -82,6 +82,7 @@ async fn register(
     jar: CookieJar,
     Json(input): Json<Credentials>,
 ) -> AppResult<impl IntoResponse> {
+    check_auth_rate_limit(&state, "register")?;
     let username = input.username.trim();
     validate_username(username)?;
     validate_password(&input.password)?;
@@ -97,6 +98,7 @@ async fn login(
     jar: CookieJar,
     Json(input): Json<Credentials>,
 ) -> AppResult<impl IntoResponse> {
+    check_auth_rate_limit(&state, "login")?;
     let username = input.username.trim();
     if username.is_empty() || input.password.is_empty() {
         return Err(AppError::InvalidInput(
@@ -172,6 +174,7 @@ async fn change_password(
     jar: CookieJar,
     Json(input): Json<ChangePassword>,
 ) -> AppResult<impl IntoResponse> {
+    check_auth_rate_limit(&state, "change_password")?;
     if !verify_password(&input.current_password, &user.password_hash) {
         return Err(AppError::Unauthenticated);
     }
@@ -332,6 +335,33 @@ fn build_expired_cookie(cfg: &AuthConfig) -> Cookie<'static> {
     builder.build()
 }
 
+/// Consume one token from the shared auth rate limiter. Called at the
+/// start of `register`, `login`, and `change_password` so credential
+/// stuffing / spraying / username-probe loops are throttled by the
+/// configured budget (default 5/sec with a 10-request burst).
+///
+/// All three endpoints share one bucket — they all expose the same
+/// argon2-verify-or-create work and the same enumeration channels, so
+/// any one of them in a tight loop should trip the limit. `endpoint`
+/// is included in the rate-limit-hit log line so operators can tell
+/// which endpoint is being probed.
+fn check_auth_rate_limit(state: &AppState, endpoint: &'static str) -> AppResult<()> {
+    use crate::auth::rate_limit::AcquireResult;
+    match state.auth_limiter.try_acquire() {
+        AcquireResult::Allowed => Ok(()),
+        AcquireResult::Denied { retry_after_secs } => {
+            tracing::warn!(
+                endpoint,
+                retry_after_secs,
+                "auth rate limit hit; returning 429"
+            );
+            Err(AppError::TooManyRequests {
+                retry_after_secs: Some(retry_after_secs),
+            })
+        }
+    }
+}
+
 fn validate_username(u: &str) -> AppResult<()> {
     if u.is_empty() {
         return Err(AppError::InvalidInput("username is required".into()));