feat: add human detection scoring and webhook notifications

Implement phase 2.1 (human detection) and 2.2 (notifications):

- Detection scorer computes 0.0-1.0 human likelihood from keystroke
  timing variance, special key usage, typing speed, command diversity,
  and session duration
- Webhook notifier sends JSON POST to configured endpoints with
  deduplication, custom headers, and event filtering
- RecordingChannel gains an event callback for feeding keystrokes
  to the scorer without coupling shell and detection packages
- Server wires scorer into session lifecycle with periodic updates
  and threshold-based notification triggers
- Web UI shows human score in session tables with highlighting
- New config sections: [detection] and [[notify.webhooks]]

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

internal/web/templates/dashboard.html

@@ -70,6 +70,7 @@
                 <th>IP</th>
                 <th>Username</th>
                 <th>Shell</th>
+                <th>Score</th>
                 <th>Connected</th>
                 <th>Disconnected</th>
             </tr>
@@ -81,11 +82,12 @@
                 <td>{{.IP}}</td>
                 <td>{{.Username}}</td>
                 <td>{{.ShellName}}</td>
+                <td>{{if .HumanScore}}{{if gt (derefFloat .HumanScore) 0.6}}<mark>{{formatScore .HumanScore}}</mark>{{else}}{{formatScore .HumanScore}}{{end}}{{else}}-{{end}}</td>
                 <td>{{formatTime .ConnectedAt}}</td>
                 <td>{{if .DisconnectedAt}}{{formatTime (derefTime .DisconnectedAt)}}{{else}}<mark>active</mark>{{end}}</td>
             </tr>
             {{else}}
-            <tr><td colspan="6">No sessions</td></tr>
+            <tr><td colspan="7">No sessions</td></tr>
             {{end}}
         </tbody>
     </table>