feat(crawler): coordinated browser restart gate in BrowserManager

Adds a Healthy/Draining/Restarting lifecycle gate. `acquire()` parks while
a restart is in progress; `coordinated_restart(deadline)` blocks new
acquires, drains in-flight leases (bounded, then forces), closes +
relaunches Chromium (re-running on_launch → re-inject session + probe),
then resumes parked acquirers. Concurrent restart requests collapse into
one relaunch; the phase always returns to Healthy so a failed relaunch
never wedges acquisition.

The metadata pass cooperates via is_restart_pending() at its per-manga
checkpoint, yielding its long-lived lease (recovery sweep next tick)
instead of stalling the drain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/crawler/pipeline.rs

@@ -192,6 +192,17 @@ pub async fn run_metadata_pass(
             }
         };
         for r in batch {
+            // Cooperative checkpoint: if a coordinated browser restart is
+            // pending, yield our (long-lived) lease so the drain can
+            // proceed instead of stalling for the rest of the walk. The
+            // pass exits un-clean, so the next tick recovery-sweeps the
+            // tail we didn't reach.
+            if browser_manager.is_restart_pending() {
+                tracing::info!(
+                    "metadata pass: browser restart pending — yielding (recovery sweep next tick)"
+                );
+                break 'outer;
+            }
             if max_refs.map(|m| stats.discovered >= m).unwrap_or(false) {
                 hit_limit = true;
                 tracing::info!(cap = ?max_refs, "max_results reached; halting walk");