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/api/dto.go

@@ -55,13 +55,39 @@ type SyncPairResponse struct {
 }
 
 type JobResponse struct {
-	ID          int64   `json:"id"`
-	SyncPairID  int64   `json:"sync_pair_id"`
-	TriggerType string  `json:"trigger_type"`
-	Status     string  `json:"status"`
-	StartedAt   *string `json:"started_at"`
-	FinishedAt  *string `json:"finished_at"`
-	LogFile    *string `json:"log_file"`
+	ID               int64   `json:"id"`
+	SyncPairID       int64   `json:"sync_pair_id"`
+	TriggerType      string  `json:"trigger_type"`
+	Status           string  `json:"status"`
+	StartedAt        *string `json:"started_at"`
+	FinishedAt       *string `json:"finished_at"`
+	LogFile          *string `json:"log_file"`
+	DurationSeconds  *int64  `json:"duration_seconds,omitempty"`
+	LogLineCount     *int64  `json:"log_line_count,omitempty"`
+}
+
+type LogLineResponse struct {
+	ID        int64  `json:"id"`
+	JobID     int64  `json:"job_id"`
+	Stream    string `json:"stream"`
+	Content   string `json:"content"`
+	Timestamp string `json:"timestamp"`
+}
+
+type SSHKeyRequest struct {
+	Label     string `json:"label"`
+	Generate  bool   `json:"generate"`
+	PublicKey string `json:"public_key"`
+}
+
+type SSHKeyResponse struct {
+	ID            int64  `json:"id"`
+	Label         string `json:"label"`
+	PublicKey     string `json:"public_key"`
+	Fingerprint   string `json:"fingerprint"`
+	InUse         bool   `json:"in_use"`
+	HasPrivateKey bool   `json:"has_private_key"`
+	CreatedAt     string `json:"created_at"`
 }
 
 type ErrorResponse struct {