Phase A-E: stability, security, observability, and test coverage

Phase A - Stability:
- Engine.Start(): recovers orphaned jobs (running/queued/waking_up) after crash
- Engine.Stop(): graceful shutdown - cancels all in-flight jobs and waits
- Queue keyed by jobID (not syncPairID): cancel now targets exact job
- Local rsync uses jobCtx (context.Background() replaced)
- Migrations wrapped in transactions; checksums stored

Phase B - Security:
- admin/admin default removed: SYNCSERVER_ADMIN_PASSWORD required on first run
- Path validation: rejects .., leading -, null bytes in sync pair paths
- Rsync flags allowlist: dangerous flags blocked (--rsync-path, -e, --files-from)
- Shell concat in RunRemote replaced with proper sh -c escaping
- knownhosts: replaced custom parser with golang.org/x/crypto/ssh/knownhosts
- RequireAdmin wired: machine CRUD, SSH key ops, settings require admin role
- deploy-keys: uses authorized_keys only (no private key upload)
- Hardcoded /var/lib/syncserver/ssh paths replaced with cfg.SSHDir()

Phase C - Operational:
- /readyz health check: DB query + SSH dir accessibility
- /metrics endpoint: Prometheus text format (jobs, queue, machines)
- Event struct JSON tags: job_id, machine_id, type (snake_case)
- EventBus broadcast: fanned out to all subscribers
- SQLite VACUUM INTO backup: scheduled before cleanup if BackupDir set
- Filesystem job log cleanup: removes .log files for purged jobs
- Backup retention: old backups auto-purged

Phase D - Frontend:
- Schedules page: REST API + full CRUD UI for cron schedules
- Dashboard: cancel button for running/queued jobs
- JobDetail: server-side log download via API
- Settings: displays data_dir from server
- 404 page: proper NotFound component

Phase E - Tests:
- auth_test.go: JWT, bcrypt, middleware, seed (18 tests)
- models_test.go: Job, Machine, SyncPair, Schedule repos (18 tests)
- go test -race: no data races found

internal/syncengine/queue_test.go

@@ -7,7 +7,7 @@ import (
 func TestQueue(t *testing.T) {
 	q := NewQueue()
 
-	if q.IsRunning(1) {
+	if q.IsRunning(100) {
 		t.Error("queue should be empty")
 	}
 
@@ -16,35 +16,37 @@ func TestQueue(t *testing.T) {
 
 	err := q.Enqueue(1, 100, cancel)
 	if err != nil {
-		t.Errorf("Enqueue(1) unexpected error: %v", err)
+		t.Errorf("Enqueue(1, 100) unexpected error: %v", err)
 	}
 
-	if !q.IsRunning(1) {
-		t.Error("queue should contain syncPair 1")
+	if !q.IsRunning(100) {
+		t.Error("queue should contain job 100")
 	}
 
-	jobID, ok := q.GetJobID(1)
+	jobID, ok := q.GetByPair(1)
 	if !ok || jobID != 100 {
-		t.Errorf("GetJobID(1) = %d, %v, want 100, true", jobID, ok)
+		t.Errorf("GetByPair(1) = %d, %v, want 100, true", jobID, ok)
 	}
 
 	err = q.Enqueue(1, 200, nil)
 	if err != ErrAlreadyRunning {
-		t.Errorf("Enqueue(1) again = %v, want ErrAlreadyRunning", err)
+		t.Errorf("Enqueue(1, 200) = %v, want ErrAlreadyRunning", err)
 	}
 
-	q.Cancel(1, true)
+	ok = q.Cancel(100, true)
+	if !ok {
+		t.Error("Cancel(100) should return true")
+	}
 	if !cancelCalled {
 		t.Error("Cancel should have called the cancel func")
 	}
-	if !q.IsCancelledByUser(1) {
-		t.Error("IsCancelledByUser should return true after Cancel(1, true)")
+	if !q.IsCancelledByUser(100) {
+		t.Error("IsCancelledByUser should return true after Cancel(100, true)")
 	}
 
-	q.Dequeue(1)
+	q.Dequeue(100)
 
-	q.Dequeue(1)
-	if q.IsRunning(1) {
+	if q.IsRunning(100) {
 		t.Error("queue should be empty after Dequeue")
 	}
 }