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)

web/src/App.tsx

@@ -5,7 +5,9 @@ import Dashboard from './pages/Dashboard';
 import Machines from './pages/Machines';
 import SyncPairs from './pages/SyncPairs';
 import JobHistory from './pages/JobHistory';
+import JobDetail from './pages/JobDetail';
 import Settings from './pages/Settings';
+import SSHKeys from './pages/SSHKeys';
 
 function ProtectedRoute({ children }: { children: JSX.Element }) {
   const [authed, setAuthed] = useState<boolean | null>(null);
@@ -27,7 +29,9 @@ export default function App() {
         <Route path="/machines" element={<ProtectedRoute><Machines /></ProtectedRoute>} />
         <Route path="/sync-pairs" element={<ProtectedRoute><SyncPairs /></ProtectedRoute>} />
         <Route path="/jobs" element={<ProtectedRoute><JobHistory /></ProtectedRoute>} />
+        <Route path="/jobs/:id" element={<ProtectedRoute><JobDetail /></ProtectedRoute>} />
         <Route path="/settings" element={<ProtectedRoute><Settings /></ProtectedRoute>} />
+        <Route path="/ssh-keys" element={<ProtectedRoute><SSHKeys /></ProtectedRoute>} />
         <Route path="*" element={<Navigate to="/" />} />
       </Routes>
     </BrowserRouter>