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/config/config.go

@@ -116,25 +116,7 @@ func Save(path string, cfg *Config) error {
 
 func (c *Config) Validate() error {
 	c.MediaPath = NormalizeMediaPath(c.MediaPath)
-	if c.MediaPath != "" {
-		info, err := os.Stat(c.MediaPath)
-		if err != nil {
-			return errors.New("media_path is not accessible")
-		}
-		if !info.IsDir() {
-			return errors.New("media_path must be a directory")
-		}
-	}
 	c.Sources = NormalizeSources(c.Sources, c.MediaPath)
-	for _, source := range c.Sources {
-		info, err := os.Stat(source.Path)
-		if err != nil {
-			return errors.New("source path is not accessible: " + source.Path)
-		}
-		if !info.IsDir() {
-			return errors.New("source path must be a directory: " + source.Path)
-		}
-	}
 	if c.ReserveFreeGB < 0 {
 		return errors.New("reserve_free_gb must be >= 0")
 	}