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/config.rs

@@ -22,6 +22,7 @@ pub struct AuthConfig {
     pub cookie_secure: bool,
     pub cookie_domain: Option<String>,
     pub session_ttl_days: i64,
+    pub rate_limit: crate::auth::rate_limit::RateLimitConfig,
 }
 
 impl Default for AuthConfig {
@@ -30,6 +31,11 @@ impl Default for AuthConfig {
             cookie_secure: true,
             cookie_domain: None,
             session_ttl_days: 30,
+            // Disabled by default so the test harness inherits a
+            // non-throttling limiter. Production `from_env` overrides
+            // to the [`PRODUCTION_PER_SEC`]/[`PRODUCTION_BURST`]
+            // defaults.
+            rate_limit: crate::auth::rate_limit::RateLimitConfig::default(),
         }
     }
 }
@@ -145,6 +151,16 @@ impl Config {
                     .ok()
                     .filter(|s| !s.is_empty()),
                 session_ttl_days: env_i64("SESSION_TTL_DAYS", 30),
+                rate_limit: crate::auth::rate_limit::RateLimitConfig {
+                    per_sec: env_u64(
+                        "AUTH_RATE_PER_SEC",
+                        crate::auth::rate_limit::PRODUCTION_PER_SEC.into(),
+                    ) as u32,
+                    burst: env_u64(
+                        "AUTH_RATE_BURST",
+                        crate::auth::rate_limit::PRODUCTION_BURST.into(),
+                    ) as u32,
+                },
             },
             upload: UploadConfig {
                 max_request_bytes: env_usize("MAX_REQUEST_BYTES", 200 * 1024 * 1024),