fix(admin): cascade base_url to opds/api URLs, fix pending registration time format

- UpdateSystemConfiguration: when base_url changes, automatically
  update opds_base_url and api_base_url derived configs
- convertPending: format time.Time as RFC3339 string instead of
  relying on string type assertion which would panic

internal/router/helpers.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"log"
 	"net/url"
+	"time"
 
 	"bookhoard/internal/database"
 	"bookhoard/templates"
@@ -66,7 +67,7 @@ func convertPending(pending []map[string]interface{}) []templates.PendingRegistr
 			RegistrationID: p["registration_id"].(string),
 			DeviceName:     p["device_name"].(string),
 			DeviceType:     p["device_type"].(string),
-			ExpiresAt:      p["expires_at"].(string),
+			ExpiresAt:      p["expires_at"].(time.Time).Format(time.RFC3339),
 		}
 	}
 	return result