feat(transcode): optimize + on-the-fly quality + HLS streaming (§6.6)

POST /tracks/{id}/optimize enqueues a transcode; GET /stream/{id}?quality=
serves a cached Opus rendition (miss → master + background warm); GET
/stream/{id}/hls/{playlist.m3u8,segment} serves the worker-generated HLS
rendition (AAC-in-TS), with ?token= propagated onto segment URLs for players
that can't set headers. Hexagonal: Transcoder port, FfmpegTranscoder adapter,
TranscodeService + cache-path helpers, transcode_track worker (idempotent),
schemas + deps wiring. ffmpeg already in the image.

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

app/domain/ports.py

@@ -497,6 +497,16 @@ class CoverArtProvider(Protocol):
     async def fetch_release_group(self, release_group_mbid: str) -> CoverArt | None: ...
 
 
+class Transcoder(Protocol):
+    """Transcodes an audio file with ffmpeg (plan §6.6 / Group B). ``to_opus``
+    writes a single Opus rendition; ``to_hls`` writes an HLS playlist + segments
+    (AAC-in-TS) into ``out_dir``. Both raise ``TranscodeError`` on failure; heavy
+    work always runs in a worker, never the request cycle."""
+
+    async def to_opus(self, src: Path, dest: Path, *, bitrate_kbps: int) -> None: ...
+    async def to_hls(self, src: Path, out_dir: Path, *, bitrate_kbps: int) -> None: ...
+
+
 class LyricsProvider(Protocol):
     """Fetches lyrics from an external database (LRCLIB) by artist/title/album/
     duration. Returns a hit or ``None`` (no match / service down), never raising."""