feat: show mining frame toast on OSD and speed up stats session deletes

- Flash mined-frame screenshot as in-overlay image toast for OSD card notifications
- Batch cover art from stored DB blobs via POST /api/stats/covers (no extra AniList fetches)
- Show delete progress toast on stats home and sessions pages
- Refresh only affected rollups on session delete instead of full rebuild
- Rebuild lifetime summaries with aggregate SQL CTEs instead of per-session loop

docs-site/anki-integration.md

@@ -220,6 +220,8 @@ Animated AVIF requires an AV1 encoder (`libaom-av1`, `libsvtav1`, or `librav1e`)
 }
 ```
 
+`notificationType` controls how a mined-card confirmation is shown: `osd` draws an on-screen message over the video, `system` posts a desktop notification, `both` does both, and `none` stays silent. The `osd` (and `both`) path also flashes a small screenshot of the mined frame in the overlay, matching the picture shown on the system notification.
+
 `overwriteAudio` applies to automatic card updates and duplicate-card enrichment. Manual clipboard subtitle updates (`Ctrl/Cmd+C`, then `Ctrl/Cmd+V`) always replace generated sentence audio, while leaving the word audio field unchanged.
 
 ## AI Translation