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/admin_library.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"strconv"
 	"strings"
 
 	"github.com/google/uuid"
@@ -378,3 +379,39 @@ func getAdminStats(ctx context.Context, cfg *Config) templates.AdminStats {
 
 	return stats
 }
+
+func registerAdminSettingsRoutes(cfg *Config, frontendProtected *echo.Group) {
+	g := frontendProtected.Group("", handlers.AdminMiddleware)
+
+	g.PUT("/admin/settings/scan", func(c *echo.Context) error {
+		ctx := c.Request().Context()
+
+		autoScan := c.FormValue("auto_scan_enabled") == "true"
+		intervalStr := c.FormValue("scan_poll_interval_seconds")
+		interval, err := strconv.Atoi(intervalStr)
+		if err != nil || interval < 1 || interval > 3600 {
+			return c.HTML(http.StatusBadRequest, `<div class="text-sm" style="color: var(--status-danger);">Interval must be between 1 and 3600 seconds</div>`)
+		}
+
+		autoScanStr := "false"
+		if autoScan {
+			autoScanStr = "true"
+		}
+		_ = cfg.Queries.UpdateSystemSetting(ctx, database.UpdateSystemSettingParams{
+			SettingKey:   "auto_scan_enabled",
+			SettingValue: autoScanStr,
+		})
+		_ = cfg.Queries.UpdateSystemSetting(ctx, database.UpdateSystemSettingParams{
+			SettingKey:   "scan_poll_interval_seconds",
+			SettingValue: strconv.Itoa(interval),
+		})
+
+		scanSettings := templates.ScanSettingsData{
+			AutoScanEnabled:         autoScan,
+			ScanPollIntervalSeconds: interval,
+		}
+		var buf bytes.Buffer
+		_ = templates.ScanSettingsSection(scanSettings).Render(ctx, &buf)
+		return c.HTML(http.StatusOK, buf.String())
+	})
+}