Revert to Docker-only source paths, fix config validation, improve transcoding info

- handlers_sources.go: revert to relative paths rooted at /media (remove standalone absolute-path mode)
- settings.html: remove manual path input, restore auto-loading source tree from /media
- config.go: remove filesystem existence checks from Validate() — paths may be temporarily unavailable
- transcoder.go: always specify fps in ffmpeg args when MaxFPS is set, preserving source fps if lower than limit
- copier.go: include source codec/format and target codec/format in transcoding task message

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

internal/copier/copier.go

@@ -392,9 +392,15 @@ func (c *Copier) processVideo(ctx context.Context, taskID string, database *db.D
 	ext := transcoder.OutputExt(profile.OutputFormat)
 	dstTranscoded := strings.TrimSuffix(dst, filepath.Ext(dst)) + ext
 
+	srcFPS := fmt.Sprintf("%.2f", info.FPS)
+	msg := fmt.Sprintf("Transcoding %s (%s/%dch/%sfps → %s/%s/%dfps %s)",
+		filepath.Base(src),
+		info.Codec, info.AudioChannels, srcFPS,
+		profile.VideoCodec, profile.AudioCodec, profile.MaxFPS, profile.OutputFormat,
+	)
 	c.tasks.Update(taskID, func(t *task.Task) {
 		t.Phase = task.PhaseTranscoding
-		t.Message = "Transcoding " + filepath.Base(src)
+		t.Message = msg
 	})
 	if t, ok := c.tasks.Get(taskID); ok {
 		_ = database.UpdateTask(*t)