feat: add scheduled scans (cron-like recurring scans)

- New `scheduled_scans` table with daily/weekly/monthly frequencies
- asyncio background scheduler loop checks for due schedules every 60s
- 6 REST endpoints: CRUD + toggle enabled + run-now
- `scheduled_scan_id` FK added to scans table; migrated automatically
- Frontend: Schedules page (list + create form), Schedules nav link,
  "Scheduled" badge on ScanDetails when scan was triggered by a schedule

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flight-comparator/frontend/src/pages/ScanDetails.tsx

@@ -1,9 +1,10 @@
 import { Fragment, useEffect, useState } from 'react';
-import { useParams, useNavigate } from 'react-router-dom';
+import { useParams, useNavigate, Link } from 'react-router-dom';
 import {
   ArrowLeft,
   PlaneTakeoff,
   Calendar,
+  CalendarClock,
   Users,
   Armchair,
   Clock,
@@ -221,6 +222,16 @@ export default function ScanDetails() {
             <h1 className="text-xl font-semibold text-on-surface">
               {scan.origin} → {scan.country}
             </h1>
+            {scan.scheduled_scan_id != null && (
+              <Link
+                to={`/schedules`}
+                className="inline-flex items-center gap-1 px-2 py-0.5 rounded-full text-xs font-medium bg-primary-container text-on-primary-container hover:opacity-80 transition-opacity"
+                title={`Scheduled scan #${scan.scheduled_scan_id}`}
+              >
+                <CalendarClock size={11} aria-hidden="true" />
+                Scheduled
+              </Link>
+            )}
           </div>
           <StatusChip status={scan.status as ScanStatus} />
         </div>