hls: rewire queue + generator to write hash-keyed playlists

Switches the watcher → VideoPlaylistManager → PlaylistGenerator path
from the basename-keyed layout
(`$VIDEO_PATH/{basename}.m3u8`) to the hash-keyed layout
(`$VIDEO_PATH/{hash[..2]}/{hash}/playlist.m3u8`) introduced in the
prior commit. Source videos that share a basename across libraries
(or across subdirs of one library) no longer overwrite each other's
playlists. The legacy HTTP endpoints in `/video/generate` /
`/video/stream` still use the basename layout — those move in a
follow-up commit alongside the stable streaming URL.

actors.rs:
- `QueueVideosMessage.video_paths: Vec<PathBuf>` →
  `videos: Vec<VideoToQueue>`. The queue handler dedups against the
  hash-keyed playlist + sentinel and forwards `GeneratePlaylistMessage`
  carrying the hash.
- `GeneratePlaylistMessage` now carries `content_hash: String`; the
  legacy `playlist_path: String` field is gone.
- `PlaylistGenerator` takes a `video_dir: PathBuf` at construction,
  computes the hash dir + playlist + sentinel + segment template via
  `hls_paths`, `mkdir -p`s the shard/hash dir before ffmpeg runs, and
  cleans up partial output on failure by walking the hash dir.
- `ScanDirectoryMessage` and its handler are retired entirely; their
  startup-walk role is taken over by the watcher's first tick (see
  `watcher.rs` below). Dropping it avoids threading an `ExifDao` into
  `VideoPlaylistManager` just so the actor can resolve hashes.
- Legacy `playlist_file_for` / `playlist_unsupported_sentinel` are
  retained behind `#[allow(dead_code)]` for the upcoming migration
  pass that retires pre-content-hash output.

watcher.rs:
- `process_new_files` keeps `content_hash` in the EXIF-batch result
  (formerly threw it away). Videos with `image_exif.content_hash =
  NULL` — mid-backfill rows — are skipped this tick rather than
  falling back to a basename-colliding playlist; they get picked up
  after `backfill_unhashed_backlog` populates the hash on a
  subsequent tick. Skipped count is logged at debug.
- The video staleness check now uses `hls_paths::playlist_for_hash`
  instead of `$VIDEO_PATH/{basename}.m3u8`.
- `last_full_scan` initialises to `UNIX_EPOCH` so the watcher's first
  tick is treated as a full scan. That covers the catch-up gap left
  by removing `ScanDirectoryMessage` — every library's existing media
  is checked once at watcher boot (≈60s after startup) instead of
  waiting up to `WATCH_FULL_INTERVAL_SECONDS` (1h default).

main.rs: removes the `ScanDirectoryMessage` import and the per-library
`do_send` loop, with a comment pointing at the watcher's first-tick
behavior.

state.rs: `PlaylistGenerator::new` now takes the video dir.

Tests: existing `video::hls_paths` (4) and `watcher::tests` (4) pass.
The basename-keyed `/video/generate` endpoint still compiles and
serves; behavior change there is deferred to the follow-up commit.

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

src/state.rs

@@ -111,7 +111,7 @@ impl AppState {
             "AppState::new requires at least one library"
         );
         let base_path = libraries_vec[0].root_path.clone();
-        let playlist_generator = PlaylistGenerator::new();
+        let playlist_generator = PlaylistGenerator::new(video_path.clone());
         let video_playlist_manager =
             VideoPlaylistManager::new(video_path.clone(), playlist_generator.start());