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

internal/router/frontend.go

@@ -995,21 +995,37 @@ func registerFrontendRoutes(cfg *Config) {
 			return renderErrorPage(c, "Error loading user", "user_load_error")
 		}
 
+		ctx := c.Request().Context()
+
 		// Fetch current system configuration - just base_url
-		baseURL := cfg.getBaseURL(c.Request().Context())
+		baseURL := cfg.getBaseURL(ctx)
 
 		systemConfig := map[string]string{
 			"base_url":         baseURL,
 			"default_timezone": "UTC",
 		}
 
-		defaultTimezone, err := cfg.Queries.GetSystemTimezone(c.Request().Context())
+		defaultTimezone, err := cfg.Queries.GetSystemTimezone(ctx)
 		if err == nil && defaultTimezone != "" {
 			systemConfig["default_timezone"] = defaultTimezone
 		}
 
+		// Fetch scan settings
+		scanSettings := templates.ScanSettingsData{
+			AutoScanEnabled:         true,
+			ScanPollIntervalSeconds: 60,
+		}
+		if val, err := cfg.Queries.GetSystemSetting(ctx, "auto_scan_enabled"); err == nil {
+			scanSettings.AutoScanEnabled = val == "true"
+		}
+		if val, err := cfg.Queries.GetSystemSetting(ctx, "scan_poll_interval_seconds"); err == nil {
+			if n, err := strconv.Atoi(val); err == nil {
+				scanSettings.ScanPollIntervalSeconds = n
+			}
+		}
+
 		var buf bytes.Buffer
-		err = templates.AdminSettings(user, systemConfig, "").Render(c.Request().Context(), &buf)
+		err = templates.AdminSettings(user, systemConfig, scanSettings, "").Render(ctx, &buf)
 		if err != nil {
 			return err
 		}
@@ -1053,6 +1069,8 @@ func registerFrontendRoutes(cfg *Config) {
 
 	// Admin library HTMX endpoints
 	registerAdminLibraryRoutes(cfg, frontendProtected)
+	// Admin settings HTMX endpoints
+	registerAdminSettingsRoutes(cfg, frontendProtected)
 
 	// ============================================================================
 	// LEGACY API ROUTES (for backward compatibility)