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/testconn.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"net"
 	"os"
-	"path/filepath"
 	"strings"
 	"time"
 
@@ -40,24 +39,17 @@ func dialSSH(ctx context.Context, host string, port int, user, privKeyPath, know
 
 	var capturedFingerprint string
 	var capturedPubKey ssh.PublicKey
+
+	callback, err := NewKnownHostsCallback(knownHostsPath, strictHostKeyChecking)
+	if err != nil {
+		return nil, "", nil, fmt.Errorf("creating host key callback: %w", err)
+	}
+
 	hostKeyCallback := func(hostname string, remote net.Addr, key ssh.PublicKey) error {
 		h := sha256.Sum256(key.Marshal())
 		capturedFingerprint = "SHA256:" + base64.RawStdEncoding.EncodeToString(h[:])
 		capturedPubKey = key
-		if strictHostKeyChecking && knownHostsPath != "" {
-			kh, err := GetKnownHost(filepath.Dir(knownHostsPath), host, port)
-			if err != nil {
-				return fmt.Errorf("checking known_hosts: %w", err)
-			}
-			if kh == nil {
-				return fmt.Errorf("host key not found in known_hosts: %s", hostname)
-			}
-			wantFP := kh.Fingerprint
-			if capturedFingerprint != wantFP {
-				return fmt.Errorf("host key mismatch: got %s, want %s", capturedFingerprint, wantFP)
-			}
-		}
-		return nil
+		return callback(hostname, remote, key)
 	}
 
 	cfg := &ssh.ClientConfig{