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/db/migrations/0002_job_history.sql

@@ -0,0 +1,13 @@
+-- 0002_job_history.sql
+
+CREATE INDEX IF NOT EXISTS idx_job_logs_job_id ON job_logs(job_id);
+CREATE INDEX IF NOT EXISTS idx_jobs_status_created ON jobs(status, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_jobs_sync_pair_id ON jobs(sync_pair_id);
+
+CREATE TABLE IF NOT EXISTS cleanup_history (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+    deleted_before  DATETIME NOT NULL,
+    logs_purged     INTEGER NOT NULL DEFAULT 0,
+    jobs_purged     INTEGER NOT NULL DEFAULT 0,
+    executed_at     DATETIME DEFAULT CURRENT_TIMESTAMP
+);