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/api/server.go

@@ -70,6 +70,8 @@ func (s *Server) routes() {
 	s.mux.HandleFunc("POST /api/disks/{diskID}/copy/start", s.handleCopyStart)
 	s.mux.HandleFunc("POST /api/disks/{diskID}/copy/cancel", s.handleCopyCancel)
 	s.mux.HandleFunc("GET /api/tasks/{id}", s.handleTaskGet)
+	s.mux.HandleFunc("GET /api/disks/profile", s.handleGetProfile)
+	s.mux.HandleFunc("PUT /api/disks/profile", s.handlePutProfile)
 }
 
 func (s *Server) handleDashboard(w http.ResponseWriter, r *http.Request) {