fix(admin): clear session-expired flag on successful browser restart (0.53.1)

A successful `coordinated_restart` re-runs `on_launch`, which re-injects
PHPSESSID and re-probes via `verify_session_with_recircuit` — so reaching
`Ok(())` proves the session is live. But the handler never dropped the
sticky `session_expired` flag, so the admin UI continued to report
"Session Expired" and chapter workers kept idling until the operator
made a second click on "Clear expired" (or pushed a new cookie).

The fix is one line in `restart_browser`: on `Ok(())`, call
`c.session.clear_expired()`. The error path still leaves the flag set
since a failed restart means the probe didn't confirm.

Adds a focused `clear_expired_flips_sticky_flag_without_touching_session`
unit test to pin the controller-side semantic; the existing
`update_persists_and_clears_expired_then_round_trips` test continues to
cover the cookie-refresh path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/api/admin/crawler.rs

@@ -233,6 +233,13 @@ async fn restart_browser(
 ) -> AppResult<Json<RestartResponse>> {
     let c = require_crawler(&state)?;
     let result = c.browser_manager.coordinated_restart(c.drain_deadline).await;
+    // A successful coordinated_restart re-runs on_launch, which re-injects
+    // PHPSESSID and re-probes — i.e. the session is live. Drop the sticky
+    // `session_expired` flag so chapter workers stop idling without
+    // requiring a second click on "Clear expired".
+    if result.is_ok() {
+        c.session.clear_expired();
+    }
     // Push the post-restart browser phase to live subscribers immediately.
     c.status.poke();
     repo::admin_audit::insert(