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/migrations/add-qso-changes-table.js

@@ -0,0 +1,74 @@
+/**
+ * Migration: Add qso_changes table for sync job rollback
+ *
+ * This script adds the qso_changes table which tracks all QSO modifications
+ * made by sync jobs, enabling rollback functionality for failed or stale jobs.
+ */
+
+import Database from 'bun:sqlite';
+import { join, dirname } from 'path';
+import { fileURLToPath } from 'url';
+
+// ES module equivalent of __dirname
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = dirname(__filename);
+
+const dbPath = join(__dirname, '../award.db');
+const sqlite = new Database(dbPath);
+
+async function migrate() {
+  console.log('Starting migration: Add qso_changes table...');
+
+  try {
+    // Check if table already exists
+    const tableExists = sqlite.query(`
+      SELECT name FROM sqlite_master
+      WHERE type='table' AND name='qso_changes'
+    `).get();
+
+    if (tableExists) {
+      console.log('Table qso_changes already exists. Migration complete.');
+      sqlite.close();
+      return;
+    }
+
+    // Create qso_changes table
+    sqlite.exec(`
+      CREATE TABLE qso_changes (
+        id INTEGER PRIMARY KEY AUTOINCREMENT,
+        job_id INTEGER NOT NULL,
+        qso_id INTEGER,
+        change_type TEXT NOT NULL,
+        before_data TEXT,
+        after_data TEXT,
+        created_at INTEGER NOT NULL DEFAULT (strftime('%s', 'now') * 1000),
+        FOREIGN KEY (job_id) REFERENCES sync_jobs(id) ON DELETE CASCADE,
+        FOREIGN KEY (qso_id) REFERENCES qsos(id) ON DELETE CASCADE
+      )
+    `);
+
+    // Create index for faster lookups during rollback
+    sqlite.exec(`
+      CREATE INDEX idx_qso_changes_job_id ON qso_changes(job_id)
+    `);
+
+    // Create index for change_type lookups
+    sqlite.exec(`
+      CREATE INDEX idx_qso_changes_change_type ON qso_changes(change_type)
+    `);
+
+    console.log('Migration complete! Created qso_changes table with indexes.');
+  } catch (error) {
+    console.error('Migration failed:', error);
+    sqlite.close();
+    process.exit(1);
+  }
+
+  sqlite.close();
+}
+
+// Run migration
+migrate().then(() => {
+  console.log('Migration script completed successfully');
+  process.exit(0);
+});