fix: wire security headers, graceful shutdown, and settings invalidation

Lifecycle (internal/app + main.go):
- App.OnShutdown hooks run after the HTTP server drains, in
  registration order: Worker.Shutdown (background jobs now drain
  instead of dying on SIGTERM), DeviceAuthMiddleware.Stop (releases
  the rate-limiter ticker), Handler.CloseScanner (fsnotify watcher
  and scan log files)

Security headers:
- SecurityHeadersMiddleware (survivor of middleware/security.go,
  renamed security_headers.go) is now on the echo stack: nosniff,
  frame-deny, HSTS, CSP (unsafe-inline required by the sanctioned
  theme flash guard + Alpine), referrer and permissions policy

Scanner settings cache:
- settings saves (UpdateScanSettings + ApplySetting for the three
  scan keys) now invalidate MediaScanner's 30s settings cache via
  SystemSettingsHandler.SetScanSettingsChanged, so changes apply on
  the next poll instead of after the TTL
- MediaScanner.Close closes the scan logger, fixing a file-descriptor
  leak on every per-item rescan (fresh scanners never closed logs)

Client fix:
- custom-section-builder's add/remove-selection buttons used inline
  onclick attributes referencing module-scoped functions, which are
  not global in a bundled ES module: the buttons threw ReferenceError.
  Functions restored and exposed on window via declare global

Also fixes a pre-existing vet error (saveCoverImage copied
echo.Context by value, mutex included) and drops the per-key
SettingsCache.InvalidateKey variant left callerless by the
full-flush invalidation wiring.

internal/handlers/media.go

@@ -1550,7 +1550,7 @@ func (mh *MediaHandler) UpdateMediaItem(c *echo.Context) error {
 	} else if req.CoverAction == "upload" {
 		file, err := c.FormFile("cover_file")
 		if err == nil {
-			savedPath, err := mh.saveCoverImage(*c, mediaUUID, file)
+			savedPath, err := mh.saveCoverImage(c, mediaUUID, file)
 			if err != nil {
 				return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to save cover image"})
 			}
@@ -2876,7 +2876,7 @@ func jsonBytesToMap(b []byte) map[string]interface{} {
 	return result
 }
 
-func (mh *MediaHandler) saveCoverImage(c echo.Context, mediaUUID uuid.UUID, file *multipart.FileHeader) (string, error) {
+func (mh *MediaHandler) saveCoverImage(c *echo.Context, mediaUUID uuid.UUID, file *multipart.FileHeader) (string, error) {
 	src, err := file.Open()
 	if err != nil {
 		return "", fmt.Errorf("failed to open uploaded file: %w", err)
@@ -3030,7 +3030,8 @@ func (mh *MediaHandler) GetAudioChapters(c *echo.Context) error {
 // SetAudioSpeed handles PUT /api/media-items/:id/audio-speed — the
 // per-user per-book playback speed upsert (plan §8). The client SPEEDS
 // list is the real gate; the server sanity-checks the range.
-func (mh *MediaHandler) SetAudioSpeed(c *echo.Context) error {	mediaID := c.Param("id")
+func (mh *MediaHandler) SetAudioSpeed(c *echo.Context) error {
+	mediaID := c.Param("id")
 	mediaUUID, err := uuid.Parse(mediaID)
 	if err != nil {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid media item id"})