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/services/lotw.service.js

@@ -1,5 +1,5 @@
 import { db, logger } from '../config.js';
-import { qsos } from '../db/schema/index.js';
+import { qsos, qsoChanges } from '../db/schema/index.js';
 import { max, sql, eq, and, or, desc, like } from 'drizzle-orm';
 import { updateJobProgress } from './job-queue.service.js';
 import { parseADIF, normalizeBand, normalizeMode } from '../utils/adif-parser.js';
@@ -258,6 +258,12 @@ export async function syncQSOs(userId, lotwUsername, lotwPassword, sinceDate = n
           existingQSO.lotwQslRdate !== dbQSO.lotwQslRdate;
 
         if (confirmationChanged) {
+          // Record before state for rollback
+          const beforeData = JSON.stringify({
+            lotwQslRstatus: existingQSO.lotwQslRstatus,
+            lotwQslRdate: existingQSO.lotwQslRdate,
+          });
+
           await db
             .update(qsos)
             .set({
@@ -266,9 +272,28 @@ export async function syncQSOs(userId, lotwUsername, lotwPassword, sinceDate = n
               lotwSyncedAt: dbQSO.lotwSyncedAt,
             })
             .where(eq(qsos.id, existingQSO.id));
+
+          // Record after state for rollback
+          const afterData = JSON.stringify({
+            lotwQslRstatus: dbQSO.lotwQslRstatus,
+            lotwQslRdate: dbQSO.lotwQslRdate,
+          });
+
+          // Track change in qso_changes table if jobId provided
+          if (jobId) {
+            await db.insert(qsoChanges).values({
+              jobId,
+              qsoId: existingQSO.id,
+              changeType: 'updated',
+              beforeData,
+              afterData,
+            });
+          }
+
           updatedCount++;
           // Track updated QSO (CALL and DATE)
           updatedQSOs.push({
+            id: existingQSO.id,
             callsign: dbQSO.callsign,
             date: dbQSO.qsoDate,
             band: dbQSO.band,
@@ -279,10 +304,32 @@ export async function syncQSOs(userId, lotwUsername, lotwPassword, sinceDate = n
           skippedCount++;
         }
       } else {
-        await db.insert(qsos).values(dbQSO);
+        // Insert new QSO
+        const [newQSO] = await db.insert(qsos).values(dbQSO).returning();
+
+        // Track change in qso_changes table if jobId provided
+        if (jobId) {
+          const afterData = JSON.stringify({
+            callsign: dbQSO.callsign,
+            qsoDate: dbQSO.qsoDate,
+            timeOn: dbQSO.timeOn,
+            band: dbQSO.band,
+            mode: dbQSO.mode,
+          });
+
+          await db.insert(qsoChanges).values({
+            jobId,
+            qsoId: newQSO.id,
+            changeType: 'added',
+            beforeData: null,
+            afterData,
+          });
+        }
+
         addedCount++;
         // Track added QSO (CALL and DATE)
         addedQSOs.push({
+          id: newQSO.id,
           callsign: dbQSO.callsign,
           date: dbQSO.qsoDate,
           band: dbQSO.band,