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/main.rs

@@ -26,7 +26,6 @@ use crate::files::{RealFileSystem, move_file};
 use crate::service::ServiceBuilder;
 use crate::state::AppState;
 use crate::tags::*;
-use crate::video::actors::ScanDirectoryMessage;
 use log::{error, info};
 
 mod ai;
@@ -119,13 +118,12 @@ fn main() -> std::io::Result<()> {
             .unwrap();
 
         let app_state = app_data.clone();
-        for lib in &app_state.libraries {
-            app_state.playlist_manager.do_send(ScanDirectoryMessage {
-                directory: lib.root_path.clone(),
-            });
-        }
 
-        // Start file watcher with playlist manager and preview generator
+        // Start file watcher with playlist manager and preview generator.
+        // The watcher's first tick is configured to be a full scan (see
+        // `watch_files`), so every library's missing HLS playlists are
+        // queued on that first iteration — no separate startup walk
+        // needed.
         let playlist_mgr_for_watcher = app_state.playlist_manager.as_ref().clone();
         let preview_gen_for_watcher = app_state.preview_clip_generator.as_ref().clone();
         // Both background jobs read from the shared `live_libraries` lock