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/disk/disk.go

@@ -18,11 +18,12 @@ const (
 )
 
 type DiskInfo struct {
-	State      DiskState `json:"state"`
-	DiskID     string    `json:"disk_id"`
-	TotalBytes int64     `json:"total_bytes"`
-	FreeBytes  int64     `json:"free_bytes"`
-	MountPath  string    `json:"mount_path"`
+	State      DiskState    `json:"state"`
+	DiskID     string       `json:"disk_id"`
+	TotalBytes int64        `json:"total_bytes"`
+	FreeBytes  int64        `json:"free_bytes"`
+	MountPath  string       `json:"mount_path"`
+	Profile    *DiskProfile `json:"profile,omitempty"`
 }
 
 const MarkerDir = ".jukebox"
@@ -55,6 +56,9 @@ func Probe(mountPath string) (DiskInfo, error) {
 
 	info.DiskID = strings.TrimSpace(string(data))
 	info.State = DiskKnown
+	if p, err := LoadProfile(mountPath); err == nil {
+		info.Profile = p
+	}
 	return info, nil
 }
 
@@ -81,6 +85,7 @@ func InitDisk(mountPath string) (string, error) {
 	if err := os.WriteFile(idPath, []byte(id), 0o644); err != nil {
 		return "", err
 	}
+	_ = SaveProfile(mountPath, DefaultProfile())
 	return id, nil
 }