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/config/config.go

@@ -30,8 +30,10 @@ type AuthConfig struct {
 }
 
 type SchedulerConfig struct {
-	Timezone       string `yaml:"timezone" env:"SYNCSERVER_SCHEDULER_TZ" default:"UTC"`
-	RetentionDays  int    `yaml:"retention_days" env:"SYNCSERVER_RETENTION_DAYS" default:"30"`
+	Timezone           string `yaml:"timezone" env:"SYNCSERVER_SCHEDULER_TZ" default:"UTC"`
+	RetentionDays      int    `yaml:"retention_days" env:"SYNCSERVER_RETENTION_DAYS" default:"30"`
+	BackupDir          string `yaml:"backup_dir" env:"SYNCSERVER_BACKUP_DIR"`
+	BackupRetentionDays int   `yaml:"backup_retention_days" env:"SYNCSERVER_BACKUP_RETENTION_DAYS" default:"7"`
 }
 
 var globalCfg *Config