fix(crawler): relaunch chromium on CDP / nav-timeout errors (0.36.3)

BrowserManager only re-launched chromium when the cached handle was
None. A crash mid-pass left the handle Some pointing at a dead
process — every subsequent acquire returned the zombie Browser, and
every nav cascaded CDP errors until the idle reaper fired.

Add BrowserManager::invalidate(): take the inner mutex, drop the
handle (closing it if present), and signal the next acquire to
relaunch. Idempotent — invalidating an empty handle is a no-op.

Wire detection via NavError::is_likely_browser_dead and a
chain-walking anyhow_looks_browser_dead helper: substring-match
common channel/connection/transport/WebSocket markers and surface
NavError::Timeout as "presumed dead." Apply at both error
boundaries — RealChapterDispatcher::dispatch and
RealMetadataPass::run.

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

backend/src/app.rs

@@ -230,7 +230,7 @@ struct RealMetadataPass {
 #[async_trait]
 impl MetadataPass for RealMetadataPass {
     async fn run(&self) -> anyhow::Result<MetadataStats> {
-        pipeline::run_metadata_pass(
+        let result = pipeline::run_metadata_pass(
             &self.browser_manager,
             &self.db,
             self.storage.as_ref(),
@@ -242,7 +242,13 @@ impl MetadataPass for RealMetadataPass {
             &self.download_allowlist,
             self.max_image_bytes,
         )
-        .await
+        .await;
+        if let Err(e) = &result {
+            if crate::crawler::nav::anyhow_looks_browser_dead(e) {
+                self.browser_manager.invalidate().await;
+            }
+        }
+        result
     }
 }
 
@@ -273,7 +279,7 @@ impl ChapterDispatcher for RealChapterDispatcher {
                     return Ok(SyncOutcome::Skipped);
                 };
                 let lease = self.browser_manager.acquire().await?;
-                let outcome = content::sync_chapter_content(
+                let result = content::sync_chapter_content(
                     &lease,
                     &self.db,
                     self.storage.as_ref(),
@@ -286,9 +292,17 @@ impl ChapterDispatcher for RealChapterDispatcher {
                     &self.download_allowlist,
                     self.max_image_bytes,
                 )
-                .await?;
+                .await;
                 drop(lease);
-                Ok(outcome)
+                match result {
+                    Ok(outcome) => Ok(outcome),
+                    Err(e) => {
+                        if crate::crawler::nav::anyhow_looks_browser_dead(&e) {
+                            self.browser_manager.invalidate().await;
+                        }
+                        Err(e)
+                    }
+                }
             }
             // Other payload kinds aren't dispatched by this daemon yet —
             // SyncManga / SyncChapterList are handled inline by the cron's