hls: add hash-keyed path helpers + VideoToQueue type

Foundation for migrating HLS playlist output from basename-keyed
(`$VIDEO_PATH/{basename}.m3u8`) to content-hash-keyed
(`$VIDEO_PATH/{hash[..2]}/{hash}/playlist.m3u8`). The basename layout
collides whenever two source videos share a filename — common with
iPhone-style sequential naming (`IMG_NNNN.MOV`) across libraries — so
the loser's playlist gets overwritten and ffmpeg keeps re-queueing the
file every scan.

This commit adds the path layout and type plumbing without touching the
actor pipeline, watcher, or HTTP handlers yet:

- `src/video/hls_paths.rs`: `playlist_for_hash`, `sentinel_for_hash`,
  `segment_template_for_hash` built on top of `content_hash::hls_dir`,
  with constants for the filenames inside the hash dir. Unit tests
  cover the sharded layout and the playlist/sentinel/segment paths
  all landing in the same directory (so HLS relative refs resolve).
- `src/content_hash::hls_dir` un-deaded — was waiting for this branch.
- `VideoToQueue` struct in `actors.rs`: pairs a source path with its
  content hash so callers that lack a hash (rows mid-backfill) skip
  the video rather than fabricate one.
- `playlist_file_for` / `playlist_unsupported_sentinel` retained as
  migration-only helpers — they're only needed by the one-shot startup
  pass that retires pre-content-hash output.

Follow-ups (separate commits on this branch): wire `hls_paths` through
the queue handler + `PlaylistGenerator`, update the watcher's
`process_new_files` to build `VideoToQueue`, switch `/video/generate`
and `/video/stream` to resolve path→hash and return stable URLs, add
the legacy-layout migration, rewrite `cleanup_orphaned_playlists` for
the new dir shape, and surface progress via Prometheus + `/hls/stats`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/content_hash.rs

@@ -52,12 +52,9 @@ pub fn thumbnail_path(thumbs_dir: &Path, hash: &str) -> PathBuf {
 
 /// Hash-keyed HLS output directory: `<video_dir>/<hash[..2]>/<hash>/`.
 /// The playlist lives at `playlist.m3u8` inside this directory and its
-/// segments are co-located so HLS relative references Just Work.
-///
-/// Allow-dead until Branch B/C rewires the HLS pipeline to use it; the
-/// helper lives here today so Branch A's path layout decisions stay
-/// adjacent to thumbnail/legacy ones.
-#[allow(dead_code)]
+/// segments are co-located so HLS relative references Just Work. See
+/// [`crate::video::hls_paths`] for the filename constants and the
+/// per-file helpers built on this dir.
 pub fn hls_dir(video_dir: &Path, hash: &str) -> PathBuf {
     let shard = shard_prefix(hash);
     video_dir.join(shard).join(hash)