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

stats/src/components/common/DeleteProgressToast.tsx

@@ -0,0 +1,32 @@
+interface DeleteProgressToastProps {
+  /** Number of sessions currently being deleted. The toast is hidden when 0. */
+  count: number;
+}
+
+/**
+ * Fixed-position toast shown while session deletions are in flight.
+ *
+ * The per-row delete buttons are only visible on hover, so once the confirm
+ * dialog closes the user has no signal that a (potentially slow) batch delete
+ * is still running. This stays on screen, independent of hover, until the work
+ * finishes.
+ */
+export function DeleteProgressToast({ count }: DeleteProgressToastProps) {
+  if (count <= 0) return null;
+
+  return (
+    <div
+      role="status"
+      aria-live="polite"
+      className="fixed bottom-4 right-4 z-50 flex items-center gap-3 rounded-lg border border-ctp-surface1 bg-ctp-surface0 px-4 py-3 shadow-lg shadow-black/30"
+    >
+      <span
+        aria-hidden="true"
+        className="h-4 w-4 shrink-0 animate-spin rounded-full border-2 border-ctp-surface2 border-t-ctp-red"
+      />
+      <span className="text-sm text-ctp-text">
+        Deleting {count} session{count === 1 ? '' : 's'}&hellip;
+      </span>
+    </div>
+  );
+}