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/models/job.go

@@ -143,3 +143,11 @@ func (r *JobRepository) Count() (int64, error) {
 	err := r.db.QueryRow("SELECT COUNT(*) FROM jobs").Scan(&n)
 	return n, err
 }
+
+func (r *JobRepository) DeleteFinishedBefore(before time.Time) (int64, error) {
+	res, err := r.db.Exec("DELETE FROM jobs WHERE finished_at IS NOT NULL AND finished_at < ?", before)
+	if err != nil {
+		return 0, err
+	}
+	return res.RowsAffected()
+}