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/commonhandlers.go

@@ -54,6 +54,22 @@ func NewHandler(db *database.Queries, connManager *wsync.ConnectionManager, queu
 	}
 }
 
+// CloseScanner releases scanner resources (fsnotify watcher, scan log files).
+// Call once during graceful shutdown.
+func (h *Handler) CloseScanner() {
+	if h.scanner != nil {
+		_ = h.scanner.Close()
+	}
+}
+
+// InvalidateScannerSettingsCache drops the scanner's cached system settings so
+// saved scan settings take effect on the next poll instead of after the TTL.
+func (h *Handler) InvalidateScannerSettingsCache() {
+	if h.scanner != nil {
+		h.scanner.InvalidateSettingsCache()
+	}
+}
+
 // parseDate parses a date string in YYYY-MM-DD format
 func parseDate(dateStr string) time.Time {
 	if dateStr == "" {