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/core/config.py

@@ -81,6 +81,9 @@ class Settings(BaseSettings):
     # -- media / storage --------------------------------------------------
     media_path: Path = Path("/data/media")
     transcode_cache_path: Path = Path("/data/transcode-cache")
+    # ffmpeg binary for transcoding/HLS (on PATH in the image); override for a
+    # non-standard location.
+    ffmpeg_path: str = "ffmpeg"
     max_parallel_downloads: int = 2
     # How many times the download worker retries a failed fetch (yt-dlp fails
     # often) before marking the job ``failed`` — exponential backoff between tries.