music indexer: index videos (concerts/clips) in artist/album dirs

Videos (all phone-compatible .mp4, plus common containers) that live in an
artist or album folder are now indexed alongside audio:

- Move 'mp4' out of AUDIO_EXT into a new VIDEO_EXT (mp4/m4v/mkv/mov/webm/avi) —
  it was wrongly treated as an audio track before.
- ffprobeVideo captures file/title/durationSec/width/height per video.
- meta.json gains an optional `videos: IndexVideo[]`; a folder with only videos
  now still gets a meta.json. Manifest entries gain optional `videos: N`.
- Video files join the album version signature (changes bump `v` for resync).
- New `videosIndexed` status counter + resync-log line.

Location is inherent in the folder rel (always an artist/album dir), so no
extra location field is needed. MUSIC_API.md documents the videos field +
manifest count. No poster/thumbnail generation yet (folder cover is reused).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

MUSIC_API.md

@@ -65,6 +65,7 @@ GET /api/music/manifest
   "albums": {
     "Albums/AC-DC/[1980] Back in Black": { "v": "50856380f1ca8f9", "cover": true, "tracks": 10 },
     "DJ Sets/Dave Clarke":              { "v": "a1b2c3d4e5f6a7b", "cover": false, "tracks": 3 },
+    "Albums/Metallica/[1989] Live Shit": { "v": "beefbeefbeefbee", "cover": true, "tracks": 0, "videos": 2 },
     "Albums/AC-DC":                     { "v": "c0ffee1234567890", "cover": true, "tracks": 0, "disco": true }
     // …
   }
@@ -72,7 +73,9 @@ GET /api/music/manifest
 ```
 `404` if the index has never been built (see §3). Entries with **`tracks: 0`** are container folders (e.g. an
 **artist** folder). An entry with **`disco: true`** is an artist folder that has a discography — fetch its
-grouping via `/discography` (§2.4).
+grouping via `/discography` (§2.4). **`videos: N`** (optional) counts video files (concerts, clips) that live
+directly in that artist/album folder — their per-file metadata is in that folder's `meta.json` (§2.2). A folder
+may have any mix of `tracks`, `videos`, and `disco`.
 
 ### 2.2 Album metadata
 
@@ -96,11 +99,22 @@ Returns the album's `meta.json`. Sends `ETag: <v>`; a request with `If-None-Matc
       "durationSec": 312
     }
     // …
+  ],
+  "videos": [                              // present only for folders that contain video files
+    {
+      "file": "1989 - Seattle.mp4",        // filename within the folder
+      "title": "Live Shit: Seattle",       // from the container title tag, if any
+      "durationSec": 8130,
+      "width": 1280,
+      "height": 720
+    }
+    // …
   ]
 }
 ```
-All track fields except `file` are optional (absent when the tag is missing).
-To stream a track: `GET /api/music/stream?path=Music/<rel>/<file>`.
+All track/video fields except `file` are optional (absent when the tag/stream info is missing). `videos` is
+omitted entirely when the folder has none.
+To stream a track or video: `GET /api/music/stream?path=Music/<rel>/<file>` (byte-range; works for `.mp4`).
 
 ### 2.3 Cover
 
@@ -160,7 +174,7 @@ GET  /api/music/reindex/status   → IndexStatus snapshot
   "running": true,
   "startedAt": 1785034701973, "finishedAt": null,
   "foldersScanned": 45, "albumsBuilt": 12, "albumsSkipped": 3,
-  "tracksIndexed": 320, "coversSaved": 12, "discographies": 3,
+  "tracksIndexed": 320, "videosIndexed": 4, "coversSaved": 12, "discographies": 3,
   "currentPath": "Albums/AC-DC/[1980] Back in Black",
   "error": null
 }