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/services/cache.go

@@ -48,10 +48,3 @@ func (c *SettingsCache) Invalidate() {
 	c.data = make(map[string]string)
 	c.lastUpdate = time.Time{}
 }
-
-func (c *SettingsCache) InvalidateKey(key string) {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-
-	delete(c.data, key)
-}