feat(admin): add scan settings + system information to settings page

- Scanning section: auto-scan toggle (on/off) and poll interval input
  with HTMX form that updates system_settings table
- System Information section: surfaces all hardcoded constants
  (session duration, password policy, rate limits, worker pool,
  sync queue, tombstone TTL, OPDS page size, CORS, etc.)
- New ScanSettingsData type, ScanSettingsSection partial template
- New HTMX endpoint: PUT /admin/settings/scan
- registerAdminSettingsRoutes for scan settings HTMX CRUD
- Fix bookhoord→bookhoard import typo in admin_library.go

templates/types.go

@@ -66,6 +66,11 @@ type AdminStats struct {
 	DeviceCount  int
 }
 
+type ScanSettingsData struct {
+	AutoScanEnabled         bool
+	ScanPollIntervalSeconds int
+}
+
 type SeriesCardData struct {
 	Name          string
 	BookCount     int64