feat: add scheduled sync system with admin UI

Implements comprehensive scheduled sync system using node-cron with
full admin interface for configuration and monitoring.

Features:
- Configurable sync schedules with cron expressions
- Enable/disable schedules without deletion
- Manual trigger for testing
- Status monitoring (last run, next run, success/failure)
- Error tracking and display
- Incremental and full sync support
- Multiple concurrent schedules
- Admin UI with schedule management

Components:

1. Sync Scheduler Service (lib/services/sync-scheduler.ts)
   - node-cron integration for scheduling
   - Database-backed schedule configuration
   - Automatic initialization on startup
   - Prevents concurrent runs of same schedule
   - Calculates next run times
   - Tracks execution status and errors

2. Database Schema (sync_schedules table)
   - Schedule configuration storage
   - Execution history tracking
   - Last run status and errors
   - Next run calculation

3. API Endpoints
   - GET /api/sync/schedules - List all schedules
   - POST /api/sync/schedules - Create schedule
   - GET /api/sync/schedules/[id] - Get schedule
   - PATCH /api/sync/schedules/[id] - Update schedule
   - DELETE /api/sync/schedules/[id] - Delete schedule
   - POST /api/sync/schedules/[id]/trigger - Manual trigger

4. Admin UI (components/admin/SyncScheduler.tsx)
   - View all schedules with status
   - Create/edit/delete schedules
   - Enable/disable toggle
   - Manual trigger button
   - Cron expression presets
   - Real-time status updates
   - Error message display
   - Next run countdown

5. Default Schedules (created on first startup, disabled)
   - Daily Incremental: 2 AM daily (0 2 * * *)
   - Weekly Full: 3 AM Sunday (0 3 * * 0)

Admin Interface:
- New 'Schedules' tab in sync page
- Schedule cards with status badges
- Enable/disable with play/pause button
- Manual trigger with clock button
- Edit dialog with cron presets
- Create dialog for new schedules
- Real-time status (running, next run, last run)
- Success/failure indicators
- Error message alerts

Cron Features:
- Full cron expression support
- Validation before saving
- Common presets (daily, weekly, hourly)
- Next run time calculation
- Automatic schedule restart on config change

Monitoring:
- Last run timestamp
- Next run countdown (e.g., 'in 2h 15m')
- Success/failure status with icons
- Error messages for failed syncs
- Running indicator (animated badge)
- Schedule validity check

Dependencies:
- node-cron: ^3.0.3
- @types/node-cron: ^3.0.11

UI Components:
- Alert component added (components/ui/alert.tsx)
- Integrated into sync page tabs
- Responsive design

Documentation:
- Complete guide (docs/SCHEDULED_SYNCS.md)
- Cron expression reference
- Best practices
- Troubleshooting guide
- API reference
- Database schema

Use Cases:
1. Daily incremental sync for recent changes
2. Weekly full sync for data integrity
3. Custom schedules for specific needs
4. Off-peak hour automation
5. Backup for webhook failures

Benefits:
- No manual intervention required
- Consistent data freshness
- Flexible scheduling
- Easy monitoring
- Error tracking
- Manual override available

Next Steps:
1. Restart application to initialize scheduler
2. Navigate to Admin → Sync → Schedules tab
3. Enable default schedules or create custom ones
4. Monitor first runs for success
5. Adjust schedules as needed

Files Added/Modified:
- lib/services/sync-scheduler.ts (new)
- app/api/sync/schedules/route.ts (new)
- app/api/sync/schedules/[id]/route.ts (new)
- app/api/sync/schedules/[id]/trigger/route.ts (new)
- components/admin/SyncScheduler.tsx (new)
- components/ui/alert.tsx (new)
- app/admin/sync/page.tsx (modified - added Schedules tab)
- docs/SCHEDULED_SYNCS.md (new)
- package.json (node-cron added)

app/admin/sync/page.tsx

@@ -9,10 +9,11 @@ import { useState, useEffect } from 'react';
 import SyncControlPanel from '@/components/admin/SyncControlPanel';
 import SyncDashboard from '@/components/admin/SyncDashboard';
 import SyncHistoryTable from '@/components/admin/SyncHistoryTable';
+import SyncScheduler from '@/components/admin/SyncScheduler';
 import { EntityType } from '@/lib/types/sync';
 import { Button } from '@/components/ui/button';
 import { Tabs, TabsContent, TabsList, TabsTrigger } from '@/components/ui/tabs';
-import { ArrowLeft, Home, Activity, History } from 'lucide-react';
+import { ArrowLeft, Home, Activity, History, Calendar } from 'lucide-react';
 import Link from 'next/link';
 
 export default function AdminSyncPage() {
@@ -83,9 +84,9 @@ export default function AdminSyncPage() {
         isSyncing={isSyncing}
       />
 
-      {/* Tabs for Sync Status and History */}
+      {/* Tabs for Sync Status, History, and Schedules */}
       <Tabs defaultValue="status" className="w-full">
-        <TabsList className="grid w-full max-w-md grid-cols-2">
+        <TabsList className="grid w-full max-w-2xl grid-cols-3">
           <TabsTrigger value="status" className="gap-2">
             <Activity className="h-4 w-4" />
             Sync Status
@@ -94,6 +95,10 @@ export default function AdminSyncPage() {
             <History className="h-4 w-4" />
             Sync History
           </TabsTrigger>
+          <TabsTrigger value="schedules" className="gap-2">
+            <Calendar className="h-4 w-4" />
+            Schedules
+          </TabsTrigger>
         </TabsList>
         
         <TabsContent value="status" className="mt-6">
@@ -103,6 +108,10 @@ export default function AdminSyncPage() {
         <TabsContent value="history" className="mt-6">
           <SyncHistoryTable refreshKey={refreshKey} />
         </TabsContent>
+        
+        <TabsContent value="schedules" className="mt-6">
+          <SyncScheduler />
+        </TabsContent>
       </Tabs>
     </div>
   );