fix: cancel in-flight encode on SIGINT/SIGTERM via context plumbing

Thread context.Context from main through watcher, encoder, and metadata
clients so a Ctrl-C during a multi-hour encode immediately kills the
ffmpeg/ffprobe/opusenc children instead of waiting for them to finish.

- main: signal.NotifyContext replaces the manual sigChan + done goroutine
- watcher.Start: takes ctx, exits on ctx.Done(); processFn signature is
  now func(context.Context, string) error
- encoder: Transcode and every helper (extractAudio, encodeOpus,
  encodeVideo, GetMediaInfo, GetStreamLanguages, calculateZscaleWidth)
  take ctx; every exec.Command becomes exec.CommandContext so the child
  is SIGKILL'd on cancel
- metadata: FetchMovieMetadata, FetchSeriesMetadata, fetchTVMazeJSON
  take ctx and use http.NewRequestWithContext

Mover stays ctx-free intentionally: a rename is fast enough that
mid-cancel cleanup is the next-restart's problem. processFile's
deferred RemoveAll(workDir) and failToFailed still run after cancel,
so partial output dies in the work dir and the source moves to failed/.

cmd/videnc/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"context"
 	"crypto/rand"
 	"encoding/hex"
 	"flag"
@@ -61,24 +62,18 @@ func main() {
 
 	metaClient := metadata.NewClient(cfg.OMDBAPIKey)
 
-	done := make(chan struct{})
-	sigChan := make(chan os.Signal, 1)
-	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
-
-	go func() {
-		<-sigChan
-		close(done)
-	}()
+	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer cancel()
 
 	w := watcher.New(cfg.Paths.Input, 15)
-	w.Start(func(inputPath string) error {
-		return processFile(inputPath, cfg, enc, metaClient, log)
-	}, done)
+	w.Start(ctx, func(ctx context.Context, inputPath string) error {
+		return processFile(ctx, inputPath, cfg, enc, metaClient, log)
+	})
 
 	log.Info("videnc-vibe started")
 }
 
-func processFile(inputPath string, cfg *types.Config, enc *encoder.Encoder, metaClient *metadata.Client, log *logger.Logger) error {
+func processFile(ctx context.Context, inputPath string, cfg *types.Config, enc *encoder.Encoder, metaClient *metadata.Client, log *logger.Logger) error {
 	log.Info(fmt.Sprintf("Processing: %s", inputPath))
 
 	filename := filepath.Base(inputPath)
@@ -97,14 +92,14 @@ func processFile(inputPath string, cfg *types.Config, enc *encoder.Encoder, meta
 	// Cleanup the work directory on every exit path, success or failure.
 	defer os.RemoveAll(workDir)
 
-	width, height, interlaced, err := enc.GetMediaInfo(inputPath)
+	width, height, interlaced, err := enc.GetMediaInfo(ctx, inputPath)
 	if err != nil {
 		log.ErrorFile(inputPath, "Getting media info", err.Error())
 		failToFailed(inputPath, cfg.Paths.Failed, log)
 		return err
 	}
 
-	streamLangs, err := enc.GetStreamLanguages(inputPath)
+	streamLangs, err := enc.GetStreamLanguages(ctx, inputPath)
 	if err != nil {
 		log.ErrorFile(inputPath, "Getting stream languages", err.Error())
 	}
@@ -130,12 +125,12 @@ func processFile(inputPath string, cfg *types.Config, enc *encoder.Encoder, meta
 
 	var meta *types.Metadata
 	if isSeries && tvmazeID != "" && season != "" && episode != "" {
-		meta, err = metaClient.FetchSeriesMetadata(tvmazeID, season, episode)
+		meta, err = metaClient.FetchSeriesMetadata(ctx, tvmazeID, season, episode)
 		if err != nil {
 			log.ErrorFile(inputPath, "Fetching series metadata", err.Error())
 		}
 	} else if imdbID != "" {
-		meta, err = metaClient.FetchMovieMetadata(imdbID)
+		meta, err = metaClient.FetchMovieMetadata(ctx, imdbID)
 		if err != nil {
 			log.ErrorFile(inputPath, "Fetching movie metadata", err.Error())
 		}
@@ -159,7 +154,7 @@ func processFile(inputPath string, cfg *types.Config, enc *encoder.Encoder, meta
 		DeleteOrigin: deleteOrigin,
 	}
 
-	if err := enc.Transcode(inputPath, workDir, job, meta, interlaced, streamLangs); err != nil {
+	if err := enc.Transcode(ctx, inputPath, workDir, job, meta, interlaced, streamLangs); err != nil {
 		log.ErrorFile(inputPath, "Transcoding", err.Error())
 		failToFailed(inputPath, cfg.Paths.Failed, log)
 		return err