perf(downloads): avoid blocking file availability probes

docs/architecture/download-manager.md

@@ -178,6 +178,7 @@ variants, contextual buttons, and theme-aware styling.
 ## Queuing, persistence, and UX notes
 
 - Every download row writes to the shared `downloads` table with statuses (`queued`, `downloading`, `paused`, `completed`, `failed`, `canceled`) plus metadata such as `bytesDownloaded`, `totalBytes`, `errorMessage`, `requestHeaders`, `resumeValidator`, the offline-detail metadata snapshot, and Xtream identifiers. Downloads are locally owned records rather than playlist children: deleting a source retains its rows and local files, keeps them visible in the global library, and disables provider handoff until that source exists again. Startup rebuilds older tables that still carry the playlist foreign key, while additive columns use the idempotent column migrations.
+- Download-list and focused-detail reads verify completed files with asynchronous `lstat` probes in the main process. One shared probe queue permits at most four filesystem calls at once and coalesces only in-flight checks for the same path; it does not cache completed results, so external file deletion is visible on the next refresh without letting frequent progress broadcasts block Electron's main thread.
 - On startup, `download-recovery.ts` converts stale `downloading` rows with a non-empty `.part` file to `paused`, converts stale `queued` rows to `paused` while keeping any retained `.part` (a resumed download waiting behind an active one persists as `queued` with its partial), and marks stale `downloading` rows without recoverable partial bytes as `failed`.
 - Queue cancellation removes a queued task or records an active cancellation request and aborts the request when available. Pausing follows the same abort path but persists `paused` and keeps the `.part`. Retries reuse the same database entry: a failed row with a retained `filePath` resumes its `.part` through HTTP Range, otherwise the retry starts from zero. Resume appends to the existing `.part` through HTTP Range with `If-Range` validation.
 - A `.part` that cannot be deleted (locked, permission denied) never loses its database path: cancel persists `canceled` while retaining `filePath` for later cleanup, and `DOWNLOADS_REMOVE` keeps the row and answers `success: false` (surfaced as a snackbar) so retrying the remove re-attempts the deletion once the lock is released.