feat: add configurable transcoding cancellation (#4411)

* feat: add configurable transcoding cancellation

Implemented EnableTranscodingCancellation configuration option to control whether
FFmpeg transcoding processes can be interrupted when client requests are cancelled.
This addresses resource management issues on low-power hardware where transcoding
processes would accumulate and cause CPU spikes.

Key changes:
- Added EnableTranscodingCancellation bool to configuration (default: false)
- Added CLI flag --enabletranscodingcancellation and TOML/env support
- Modified FFmpeg package to always use exec.CommandContext for consistency
- Implemented conditional context handling in NewTranscodingCache function
- When enabled: uses request context directly (allows cancellation)
- When disabled: uses background context with request metadata preserved
- Added comprehensive tests for both FFmpeg and transcoding layers
- Maintained backward compatibility with existing behavior as default

The implementation follows proper layered architecture with policy decisions
at the media streaming layer and execution utilities remaining focused on
their core responsibilities.

Signed-off-by: Deluan <deluan@navidrome.org>

* test: refactor FFmpeg context cancellation tests for improved clarity and reliability

Signed-off-by: Deluan <deluan@navidrome.org>

* test: reset FFmpeg initialization

Signed-off-by: Deluan <deluan@navidrome.org>

* test: improve FFmpeg context cancellation tests for cross-platform compatibility

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

conf/configuration.go

@@ -41,6 +41,7 @@ type configOptions struct {
 	UIWelcomeMessage                string
 	MaxSidebarPlaylists             int
 	EnableTranscodingConfig         bool
+	EnableTranscodingCancellation   bool
 	EnableDownloads                 bool
 	EnableExternalServices          bool
 	EnableInsightsCollector         bool
@@ -492,6 +493,7 @@ func setViperDefaults() {
 	viper.SetDefault("uiwelcomemessage", "")
 	viper.SetDefault("maxsidebarplaylists", consts.DefaultMaxSidebarPlaylists)
 	viper.SetDefault("enabletranscodingconfig", false)
+	viper.SetDefault("enabletranscodingcancellation", false)
 	viper.SetDefault("transcodingcachesize", "100MB")
 	viper.SetDefault("imagecachesize", "100MB")
 	viper.SetDefault("albumplaycountmode", consts.AlbumPlayCountModeAbsolute)