feat: add sync job cancel and rollback with real-time updates

Implement comprehensive sync job management with rollback capabilities
and real-time status updates on the dashboard.

## Features

### Cancel & Rollback
- Users can cancel failed or stale (>1h) sync jobs
- Rollback deletes added QSOs and restores updated QSOs to previous state
- Uses qso_changes table to track all modifications with before/after snapshots
- Server-side validation prevents cancelling completed or active jobs

### Database Changes
- Add qso_changes table to track QSO modifications per job
- Stores change type (added/updated), before/after data snapshots
- Enables precise rollback of sync operations
- Migration script included

### Real-time Updates
- Dashboard now polls for job updates every 2 seconds
- Smart polling: starts when jobs active, stops when complete
- Job status badges update in real-time (pending → running → completed)
- Cancel button appears/disappears based on job state

### Backend
- Fixed job ordering to show newest first (desc createdAt)
- Track all QSO changes during LoTW/DCL sync operations
- cancelJob() function handles rollback logic
- DELETE /api/jobs/:jobId endpoint for cancelling jobs

### Frontend
- jobsAPI.cancel() method for cancelling jobs
- Dashboard shows last 5 sync jobs with status, stats, duration
- Real-time job status updates via polling
- Cancel button with confirmation dialog
- Loading state and error handling

### Logging Fix
- Changed from Bun.write() to fs.appendFile() for reliable log appending
- Logs now persist across server restarts instead of being truncated

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/backend/index.js

@@ -20,6 +20,7 @@ import {
   getJobStatus,
   getUserActiveJob,
   getUserJobs,
+  cancelJob,
 } from './services/job-queue.service.js';
 import {
   getAllAwards,
@@ -485,6 +486,42 @@ const app = new Elysia()
     }
   })
 
+  /**
+   * DELETE /api/jobs/:jobId
+   * Cancel and rollback a sync job (requires authentication)
+   * Only allows cancelling failed, completed, or stale running jobs (>1 hour)
+   */
+  .delete('/api/jobs/:jobId', async ({ user, params, set }) => {
+    if (!user) {
+      set.status = 401;
+      return { success: false, error: 'Unauthorized' };
+    }
+
+    try {
+      const jobId = parseInt(params.jobId);
+      if (isNaN(jobId)) {
+        set.status = 400;
+        return { success: false, error: 'Invalid job ID' };
+      }
+
+      const result = await cancelJob(jobId, user.id);
+
+      if (!result.success) {
+        set.status = 400;
+        return result;
+      }
+
+      return result;
+    } catch (error) {
+      logger.error('Error cancelling job', { error: error.message, userId: user?.id, jobId: params.jobId });
+      set.status = 500;
+      return {
+        success: false,
+        error: 'Failed to cancel job',
+      };
+    }
+  })
+
   /**
    * GET /api/qsos
    * Get user's QSOs (requires authentication)