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/agent/types.go

@@ -338,6 +338,11 @@ type LibrarySyncItem struct {
 	AudioTracks       any      `json:"audioTracks,omitempty"`
 	SubtitleTracks    any      `json:"subtitleTracks,omitempty"`
 	VideoInfo         any      `json:"videoInfo,omitempty"`
+	// Integrity flags a damaged / incompletely-downloaded file ("damaged" or
+	// empty). IntegrityReason is a stable code (ebml_corrupt, moov_missing,
+	// no_duration, …) the web maps to a localized "re-download" message.
+	Integrity         string   `json:"integrity,omitempty"`
+	IntegrityReason   string   `json:"integrityReason,omitempty"`
 }
 
 // LibrarySyncResponse is returned after syncing library items.