Add per-disk profiles with video transcoding support

Each disk stores .jukebox/profile.json with copy parameters (dest
folder, overwrite mode, file select, reserve space, auto-copy) and
optional transcoding limits for the target player (codec, resolution,
bitrate, FPS, audio channels, output format).

On copy, video files are probed with ffprobe; if they exceed the
profile limits they are transcoded via ffmpeg (-threads 0 for full
CPU usage), otherwise copied as-is. Scale filter never upscales.

New: internal/disk/profile.go, internal/transcoder/{detect,transcoder}.go
API: GET/PUT /api/disks/profile?mount_path=
UI: disk profile panel in dashboard for known disks
Dockerfile: adds ffmpeg to the runtime image

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/task/task.go

@@ -23,6 +23,7 @@ const (
 	PhaseReplacing      = "replacing"
 	PhaseLoadingHistory = "loading_history"
 	PhaseScanning       = "scanning"
+	PhaseTranscoding    = "transcoding"
 	PhaseCopying        = "copying"
 )