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/sshmanager/knownhosts.go

@@ -8,6 +8,7 @@ import (
 	"strings"
 
 	"golang.org/x/crypto/ssh"
+	"golang.org/x/crypto/ssh/knownhosts"
 )
 
 type KnownHost struct {
@@ -17,6 +18,23 @@ type KnownHost struct {
 	Fingerprint string
 }
 
+func NewKnownHostsCallback(knownHostsPath string, strictHostKeyChecking bool) (ssh.HostKeyCallback, error) {
+	if !strictHostKeyChecking {
+		return ssh.InsecureIgnoreHostKey(), nil
+	}
+	if knownHostsPath == "" {
+		return ssh.InsecureIgnoreHostKey(), nil
+	}
+	_, err := os.Stat(knownHostsPath)
+	if os.IsNotExist(err) {
+		return ssh.InsecureIgnoreHostKey(), nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return knownhosts.New(knownHostsPath)
+}
+
 func EnsureKnownHosts(sshDir string) (string, error) {
 	path := filepath.Join(sshDir, "known_hosts")
 	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDONLY, 0644)
@@ -27,6 +45,8 @@ func EnsureKnownHosts(sshDir string) (string, error) {
 	return path, nil
 }
 
+// AddKnownHost stores the host key in standard ssh known_hosts format (hostname keytype base64key).
+// NOTE: Existing entries in known_hosts may need to be regenerated if they were stored in a different format.
 func AddKnownHost(sshDir, host string, port int, keyData []byte) error {
 	path := filepath.Join(sshDir, "known_hosts")
 	f, err := os.OpenFile(path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)