fix: isolate per-job intermediates in paths.work and harden failure cleanup

Adds a per-job scratch directory under the new `paths.work` config (default
`./work`) so audio.<n>.wav, audio.<n>.opus and output.mkv no longer live
beside the user's sources in paths.input. The work subdir is named after
the input basename and unconditionally removed when processFile returns
(success or failure), which kills bug #4 (intermediates leaking into the
user-owned input folder; output.mkv getting re-picked by the 15-second
watcher tick on rename failure; fixed-name collision risk for any future
concurrency).

Tightens the failure paths in main.processFile too (bug #25):
- mover.MoveToFailed return values are now surfaced via a new
  failToFailed helper.
- The helper os.Stats the source first; missing -> log and skip instead
  of the previous silent no-op when MoveToFailed was called on
  output.mkv before it existed.
- On rename-output failure, the source is now routed to paths.failed
  (it was previously left in paths.input, causing an infinite re-encode
  loop on the next watcher tick). The old MoveToFailed on the work-dir
  output is dropped — the deferred RemoveAll covers it.

Mechanical changes:
- PathsConfig gains `Work string \`yaml:"work"\`` with default ./work,
  included in EnsureDirs.
- Encoder.Transcode signature now takes workDir; extractAudio,
  encodeOpus and encodeVideo all write into workDir. The internal
  cleanupWavs/cleanupOpus defers are gone (RemoveAll in main is the
  one cleanup path).
- MANUAL.md updated: example config, field reference, §6 pipeline
  step wording, §9 failure handling description, §11 runtime
  directories block.

internal/encoder/encoder.go

@@ -3,7 +3,6 @@ package encoder
 import (
 	"encoding/json"
 	"fmt"
-	"os"
 	"os/exec"
 	"path/filepath"
 	"regexp"
@@ -222,21 +221,18 @@ func (e *Encoder) calculateZscaleWidth(path string, originalHeight int) (string,
 	return zscale, newWidth, nil
 }
 
-func (e *Encoder) Transcode(input string, job *types.Job, metadata *types.Metadata, interlaced bool, streamLangs []StreamMetadata) error {
-	dir := filepath.Dir(input)
-	audioWavs, err := e.extractAudio(input, dir, streamLangs)
+func (e *Encoder) Transcode(input, workDir string, job *types.Job, metadata *types.Metadata, interlaced bool, streamLangs []StreamMetadata) error {
+	audioWavs, err := e.extractAudio(input, workDir, streamLangs)
 	if err != nil {
 		return fmt.Errorf("extracting audio: %w", err)
 	}
-	defer e.cleanupWavs(audioWavs)
 
-	opusFiles, err := e.encodeOpus(audioWavs, dir)
+	opusFiles, err := e.encodeOpus(audioWavs, workDir)
 	if err != nil {
 		return fmt.Errorf("encoding opus: %w", err)
 	}
-	defer e.cleanupOpus(opusFiles)
 
-	if err := e.encodeVideo(input, opusFiles, job, metadata, interlaced, streamLangs); err != nil {
+	if err := e.encodeVideo(input, workDir, opusFiles, job, metadata, interlaced, streamLangs); err != nil {
 		return fmt.Errorf("encoding video: %w", err)
 	}
 
@@ -257,7 +253,7 @@ func audioStreamsInSourceOrder(streamLangs []StreamMetadata) []StreamMetadata {
 	return audio
 }
 
-func (e *Encoder) extractAudio(input, dir string, streamLangs []StreamMetadata) ([]string, error) {
+func (e *Encoder) extractAudio(input, workDir string, streamLangs []StreamMetadata) ([]string, error) {
 	audio := audioStreamsInSourceOrder(streamLangs)
 	if len(audio) == 0 {
 		return nil, fmt.Errorf("no audio streams found")
@@ -265,7 +261,7 @@ func (e *Encoder) extractAudio(input, dir string, streamLangs []StreamMetadata)
 
 	wavs := make([]string, 0, len(audio))
 	for srcAudioIndex := range audio {
-		wavPath := filepath.Join(dir, fmt.Sprintf("audio.%d.wav", srcAudioIndex))
+		wavPath := filepath.Join(workDir, fmt.Sprintf("audio.%d.wav", srcAudioIndex))
 		cmd := exec.Command(e.ffmpegPath, "-i", input,
 			"-map", fmt.Sprintf("0:a:%d", srcAudioIndex),
 			"-vn", "-c:a", "pcm_s16le", "-ar", "48000",
@@ -278,22 +274,22 @@ func (e *Encoder) extractAudio(input, dir string, streamLangs []StreamMetadata)
 	return wavs, nil
 }
 
-func (e *Encoder) encodeOpus(wavs []string, dir string) ([]string, error) {
+func (e *Encoder) encodeOpus(wavs []string, workDir string) ([]string, error) {
 	var opusFiles []string
 	for _, wav := range wavs {
-		out := strings.Replace(wav, ".wav", ".opus", 1)
+		base := filepath.Base(strings.Replace(wav, ".wav", ".opus", 1))
+		out := filepath.Join(workDir, base)
 		cmd := exec.Command(e.opusencPath, "--bitrate", "128k", wav, out)
-		if out, err := cmd.CombinedOutput(); err != nil {
-			return nil, fmt.Errorf("opusenc: %s %w", out, err)
+		if logOut, err := cmd.CombinedOutput(); err != nil {
+			return nil, fmt.Errorf("opusenc: %s %w", logOut, err)
 		}
 		opusFiles = append(opusFiles, out)
 	}
 	return opusFiles, nil
 }
 
-func (e *Encoder) encodeVideo(input string, opusFiles []string, job *types.Job, metadata *types.Metadata, interlaced bool, streamLangs []StreamMetadata) error {
-	dir := filepath.Dir(input)
-	outFile := filepath.Join(dir, "output.mkv")
+func (e *Encoder) encodeVideo(input, workDir string, opusFiles []string, job *types.Job, metadata *types.Metadata, interlaced bool, streamLangs []StreamMetadata) error {
+	outFile := filepath.Join(workDir, "output.mkv")
 
 	svtParams := fmt.Sprintf("film-grain=10:film-grain-denoise=1:scd=1:qm-min=4:qm-max=15:keyint=10s")
 
@@ -398,15 +394,3 @@ func (e *Encoder) encodeVideo(input string, opusFiles []string, job *types.Job,
 
 	return nil
 }
-
-func (e *Encoder) cleanupWavs(files []string) {
-	for _, f := range files {
-		os.Remove(f)
-	}
-}
-
-func (e *Encoder) cleanupOpus(files []string) {
-	for _, f := range files {
-		os.Remove(f)
-	}
-}