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/config/config.go

@@ -28,7 +28,8 @@ type AuthConfig struct {
 }
 
 type SchedulerConfig struct {
-	Timezone string `yaml:"timezone" env:"SYNCSERVER_SCHEDULER_TZ" default:"UTC"`
+	Timezone       string `yaml:"timezone" env:"SYNCSERVER_SCHEDULER_TZ" default:"UTC"`
+	RetentionDays  int    `yaml:"retention_days" env:"SYNCSERVER_RETENTION_DAYS" default:"30"`
 }
 
 var globalCfg *Config
@@ -42,7 +43,8 @@ func Load(configPath, dataDir, addr string) (*Config, error) {
 			JWTExpiryH: 24,
 		},
 		Scheduler: SchedulerConfig{
-			Timezone: "UTC",
+			Timezone:      "UTC",
+			RetentionDays: 30,
 		},
 	}