Add SSH key management, job history persistence, and live streaming

- SSH key management: generate ed25519 keypairs or import public keys
  from UI (/ssh-keys), per-machine key selection in Machines form,
  one-time private key download with hash verification
- Fix engine to use machine-specific SSH key (was hardcoded to server key)
- Job log persistence: write to job_logs table (DB) with batched inserts,
  buffer of 50 lines; GetAllFiltered with status/pair/date range filters
- EventBus refactor: per-job subscriber channels, global channel, non-blocking
- SSE endpoints: /jobs/stream (all), /jobs/:id/log/stream (per-job live)
- JobDetail page: live log streaming, auto-scroll, cancel, duration
- JobHistory: filters (pair, status, date range), pagination, link to detail
- Cleanup scheduler: daily purge of job_logs and finished jobs older than
  SYNCSERVER_RETENTION_DAYS (default 30)
- Migration 0002: indexes on job_logs(job_id), jobs(status,created_at),
  jobs(sync_pair_id)

internal/scheduler/scheduler.go

@@ -32,6 +32,8 @@ func New(database interface{ SQLDB() *sql.DB }, engine *syncengine.Engine, cfg *
 func (s *Scheduler) Start() {
 	s.wg.Add(1)
 	go s.run()
+	s.wg.Add(1)
+	go s.cleanupRun()
 	slog.Info("scheduler started")
 }
 
@@ -93,3 +95,46 @@ func (s *Scheduler) tick() {
 		}(jobID, sch.SyncPairID, sch.ID)
 	}
 }
+
+func (s *Scheduler) cleanupRun() {
+	defer s.wg.Done()
+	ticker := time.NewTicker(24 * time.Hour)
+	defer ticker.Stop()
+
+	s.cleanup()
+	for {
+		select {
+		case <-s.stopCh:
+			return
+		case <-ticker.C:
+			s.cleanup()
+		}
+	}
+}
+
+func (s *Scheduler) cleanup() {
+	retentionDays := s.cfg.Scheduler.RetentionDays
+	if retentionDays <= 0 {
+		return
+	}
+	before := time.Now().AddDate(0, 0, -retentionDays)
+
+	logRepo := models.NewJobLogRepository(s.db)
+	jobRepo := models.NewJobRepository(s.db)
+
+	deletedLogs, err := logRepo.DeleteBefore(before)
+	if err != nil {
+		slog.Error("cleanup: failed to purge old job logs", "error", err)
+		return
+	}
+
+	deletedJobs, err := jobRepo.DeleteFinishedBefore(before)
+	if err != nil {
+		slog.Error("cleanup: failed to purge old jobs", "error", err)
+		return
+	}
+
+	if deletedLogs > 0 || deletedJobs > 0 {
+		slog.Info("cleanup: purged old records", "logs_deleted", deletedLogs, "jobs_deleted", deletedJobs, "before", before.Format("2006-01-02"))
+	}
+}