music indexer: generate video poster thumbnails

Each indexed video gets a compressed poster (a frame grab ~10% in, capped at
30s, scaled ≤600px q5 like covers), written to cache/<rel>/posters/<file>.jpg
and recorded as `poster` on the meta.videos entry. The posters dir is wiped and
regenerated on each rebuild so orphans (removed videos) don't linger. New
`postersSaved` status counter.

Served by a new sidecar route GET /api/music/poster?path=<rel>&file=<video>
(image/jpeg, ETag=<v>, 304, 404 when none) — path-safe via basename.

Verified end-to-end on a real .mp4: video-only album → manifest {tracks:0,
videos:1}, meta.poster set, 14 KB poster on disk. MUSIC_API.md documents the
poster field + endpoint + postersSaved.

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

MUSIC_API.md

@@ -106,7 +106,8 @@ Returns the album's `meta.json`. Sends `ETag: <v>`; a request with `If-None-Matc
       "title": "Live Shit: Seattle",       // from the container title tag, if any
       "durationSec": 8130,
       "width": 1280,
-      "height": 720
+      "height": 720,
+      "poster": "posters/1989 - Seattle.mp4.jpg"  // present when a poster was generated (see §2.3.1)
     }
     // …
   ]
@@ -124,6 +125,15 @@ GET /api/music/cover?path=<rel>
 Compressed JPEG (≤600px on the long edge, ~30–80 KB). Sends `ETag: <v>`; `If-None-Match: <v>` → `304`.
 Only meaningful when the manifest entry has `"cover": true`.
 
+#### 2.3.1 Video poster
+
+```
+GET /api/music/poster?path=<rel>&file=<video filename>
+```
+A compressed frame grab for a video (≤600px, same treatment as covers), taken ~10% into the clip. `file` is
+the video's filename within `<rel>` (URL-encode it). Sends `ETag: <v>`; `If-None-Match: <v>` → `304`; `404`
+when the video has no poster. Only request it when that video's `meta.videos[]` entry has a `poster` field.
+
 ### 2.4 Discography (artist album grouping)
 
 For artist folders (manifest entry with `"disco": true`), this returns a map of **album folder → release
@@ -174,7 +184,7 @@ GET  /api/music/reindex/status   → IndexStatus snapshot
   "running": true,
   "startedAt": 1785034701973, "finishedAt": null,
   "foldersScanned": 45, "albumsBuilt": 12, "albumsSkipped": 3,
-  "tracksIndexed": 320, "videosIndexed": 4, "coversSaved": 12, "discographies": 3,
+  "tracksIndexed": 320, "videosIndexed": 4, "coversSaved": 12, "postersSaved": 4, "discographies": 3,
   "currentPath": "Albums/AC-DC/[1980] Back in Black",
   "error": null
 }