feat(library): detect corrupt/incomplete files during scan

ffprobe already runs on every scanned file; now we capture its stderr and
assess integrity from it. assessIntegrity flags a file "damaged" on the
markers that mean the container/bitstream is unusable: invalid_data,
ebml_corrupt, moov_missing, bitstream_corrupt, plus no_duration (a video
stream with non-positive duration = a truncated/incomplete download).

The verdict rides on MediaInfo.Integrity (IntegrityInfo{Damaged,Reason}),
maps onto LibrarySyncItem.{Integrity,IntegrityReason}, and syncs to the web
so a damaged file can be surfaced at rest instead of only blowing up at
playback.

Bumps the scan cache version (1 → 2) so existing entries re-probe once, and
the scanner re-probes any cached entry that has no integrity verdict yet.

internal/library/scanner.go

@@ -145,11 +145,16 @@ func scanSingleFile(ctx context.Context, ffprobePath, filePath string, cacheIdx
 	// Parse season/episode
 	item.Season, item.Episode = ParseSeasonEpisode(item.FileName)
 
-	// Incremental: skip if file hasn't changed
+	// Incremental: skip if file hasn't changed. EXCEPT a previously-damaged
+	// file is always re-probed — a re-download to the same path can land with
+	// an identical size+mtime (some torrent clients preserve the torrent's
+	// mtime), so trusting the cached "damaged" verdict would pin a now-healthy
+	// file as broken forever. Re-probing damaged items is cheap (they're few).
 	if incremental && existing != nil {
 		if idx, ok := cacheIdx[filePath]; ok {
 			cached := existing.Items[idx]
-			if cached.FileSize == item.FileSize && cached.ModTime == item.ModTime && cached.MediaInfo != nil {
+			if cached.FileSize == item.FileSize && cached.ModTime == item.ModTime &&
+				cached.MediaInfo != nil && cached.MediaInfo.Integrity == nil {
 				item.MediaInfo = cached.MediaInfo
 				return item
 			}