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/error.rs

@@ -21,6 +21,11 @@ pub enum AppError {
     PayloadTooLarge(String),
     #[error("unsupported media type: {0}")]
     UnsupportedMediaType(String),
+    /// 429 with an optional `Retry-After` header value (in seconds).
+    #[error("too many requests")]
+    TooManyRequests {
+        retry_after_secs: Option<u64>,
+    },
     /// Semantic per-field validation failure. `details` is rendered into the
     /// envelope so the client can highlight the bad field(s).
     #[error("validation failed")]
@@ -51,6 +56,7 @@ impl AppError {
             AppError::Conflict(_) => "conflict",
             AppError::PayloadTooLarge(_) => "payload_too_large",
             AppError::UnsupportedMediaType(_) => "unsupported_media_type",
+            AppError::TooManyRequests { .. } => "too_many_requests",
             AppError::ValidationFailed { .. } => "validation_failed",
             AppError::Database(sqlx::Error::RowNotFound) => "not_found",
             AppError::Database(_) => "internal_error",
@@ -79,6 +85,31 @@ impl IntoResponse for AppError {
             AppError::UnsupportedMediaType(msg) => {
                 (StatusCode::UNSUPPORTED_MEDIA_TYPE, msg.clone(), None)
             }
+            AppError::TooManyRequests { retry_after_secs } => {
+                // Emit `Retry-After: N` (RFC 6585 §4) so a well-behaved
+                // client can back off correctly. Done by building the
+                // response by hand below — the `(status, headers,
+                // body)` tuple shape doesn't fit the standard
+                // `(status, body)` IntoResponse path for the other
+                // variants.
+                let body = json!({
+                    "error": {
+                        "code": code,
+                        "message": "too many requests; slow down",
+                    }
+                });
+                let mut resp = (StatusCode::TOO_MANY_REQUESTS, Json(body)).into_response();
+                if let Some(secs) = retry_after_secs {
+                    // `HeaderValue: From<u64>` skips both the
+                    // intermediate `String` allocation and the
+                    // fallible-by-shape `from_str` path.
+                    resp.headers_mut().insert(
+                        axum::http::header::RETRY_AFTER,
+                        axum::http::HeaderValue::from(*secs),
+                    );
+                }
+                return resp;
+            }
             AppError::ValidationFailed { message, details } => (
                 StatusCode::UNPROCESSABLE_ENTITY,
                 message.clone(),