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/sync.go

@@ -36,6 +36,10 @@ func BuildSyncItems(cache *LibraryCache) []agent.LibrarySyncItem {
 			si.AudioTracks = item.MediaInfo.Audio
 			si.SubtitleTracks = item.MediaInfo.Subtitles
 			si.VideoInfo = item.MediaInfo.Video
+			if integ := item.MediaInfo.Integrity; integ != nil && integ.Damaged {
+				si.Integrity = "damaged"
+				si.IntegrityReason = integ.Reason
+			}
 		}
 
 		items = append(items, si)