feat: add session replay with terminal playback via xterm.js

Persist byte-level I/O events from SSH sessions to SQLite and add a web
UI to replay them with original timing. Events are buffered in memory
and flushed every 2s to avoid blocking SSH I/O on database writes.

- Add session_events table (migration 002)
- Add SessionEvent type and storage methods (SQLite + MemoryStore)
- Change RecordingChannel to support multiple callbacks
- Add EventRecorder for buffered event persistence
- Add session detail page with xterm.js terminal replay
- Add /api/sessions/{id}/events JSON endpoint
- Linkify session IDs in dashboard and active sessions
- Vendor xterm.js v5.3.0

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/web/templates/fragments/active_sessions.html

@@ -13,7 +13,7 @@
     <tbody>
         {{range .}}
         <tr>
-            <td><code>{{truncateID .ID}}</code></td>
+            <td><a href="/sessions/{{.ID}}"><code>{{truncateID .ID}}</code></a></td>
             <td>{{.IP}}</td>
             <td>{{.Username}}</td>
             <td>{{.ShellName}}</td>