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/.

internal/watcher/watcher.go

@@ -1,6 +1,7 @@
 package watcher
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -60,23 +61,23 @@ func New(inputDir string, intervalSeconds int) *Watcher {
 	}
 }
 
-func (w *Watcher) Start(processFn func(string) error, done chan struct{}) {
+func (w *Watcher) Start(ctx context.Context, processFn func(context.Context, string) error) {
 	ticker := time.NewTicker(w.interval)
 	defer ticker.Stop()
 
-	w.scanAndProcess(processFn)
+	w.scanAndProcess(ctx, processFn)
 
 	for {
 		select {
-		case <-done:
+		case <-ctx.Done():
 			return
 		case <-ticker.C:
-			w.scanAndProcess(processFn)
+			w.scanAndProcess(ctx, processFn)
 		}
 	}
 }
 
-func (w *Watcher) scanAndProcess(processFn func(string) error) {
+func (w *Watcher) scanAndProcess(ctx context.Context, processFn func(context.Context, string) error) {
 	files, err := filepath.Glob(filepath.Join(w.inputDir, "*.mkv"))
 	if err != nil {
 		fmt.Printf("Error scanning input directory: %v\n", err)
@@ -118,7 +119,7 @@ func (w *Watcher) scanAndProcess(processFn func(string) error) {
 			continue
 		}
 
-		if err := processFn(file); err != nil {
+		if err := processFn(ctx, file); err != nil {
 			fmt.Printf("Error processing %s: %v\n", file, err)
 			nextFailed[file] = failureRecord{
 				failedAt:  now,