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/app.rs

@@ -12,6 +12,7 @@ use tokio_util::sync::CancellationToken;
 use tower_http::cors::{AllowOrigin, CorsLayer};
 use tower_http::trace::TraceLayer;
 
+use crate::auth::rate_limit::AuthRateLimiter;
 use crate::config::{AuthConfig, Config, CrawlerConfig, CrawlerModePref, UploadConfig};
 use crate::crawler::browser_manager::{self, BrowserManager};
 use crate::crawler::content::{self, SyncOutcome};
@@ -31,6 +32,10 @@ pub struct AppState {
     pub storage: Arc<dyn Storage>,
     pub auth: AuthConfig,
     pub upload: UploadConfig,
+    /// Shared rate limiter guarding the `/auth/*` mutation endpoints.
+    /// One instance per AppState so tests stay isolated across the
+    /// same process.
+    pub auth_limiter: Arc<AuthRateLimiter>,
 }
 
 /// Bundle returned by [`build`]. The router is what `axum::serve` consumes;
@@ -65,11 +70,13 @@ pub async fn build(config: Config) -> anyhow::Result<AppHandle> {
         None
     };
 
+    let auth_limiter = Arc::new(AuthRateLimiter::new(config.auth.rate_limit));
     let state = AppState {
         db,
         storage,
         auth: config.auth.clone(),
         upload: config.upload.clone(),
+        auth_limiter,
     };
     let router = router(state).layer(cors_layer(&config.cors_allowed_origins));
     Ok(AppHandle { router, daemon })