feat(p10): archivage automatique des sessions terminées (rétention configurable)

- DB: migration #10 (sessions.archived_at + index), helpers archive/unarchive/archiveExpired (soft-archive, jamais de DELETE)
- core/retention-settings.ts: session_retention_days (défaut 30, 0=jamais, 1-3650) + session_purge_days (off)
- core/session-archive.ts: scheduler start/stop/sweep (calqué DiscoveryService), câblé dans runDaemon
- pty-manager.list({includeArchived}) + SessionSummary.archived + emitHistoricalUpdate
- routes: GET /sessions?includeArchived, POST/DELETE /sessions/:id/archive, POST /sessions/archive-now
- protocole additif: message WS session_archived (relayé par la gateway, abonnés 'sessions')
- settings: retentionDays/purgeDays exposés et validés dans PATCH /settings
- web: store sessions (showArchived + archive/unarchive), SessionsListView (toggle/badge/actions), SettingsView (slider rétention), i18n EN+FR
- tests: retention-settings + session-archive (vitest) + settings-routes étendu ; acceptance-p10.mjs (sweep, event WS, resume d'une session archivée → 201, rétention=0)

packages/web/src/lib/ws-client.ts

@@ -26,7 +26,7 @@ export interface TerminalSink {
   onControlChanged(controlling: boolean): void;
 }
 
-export type SessionEvent = Extract<ServerMessage, { type: 'session_update' | 'session_exit' }>;
+export type SessionEvent = Extract<ServerMessage, { type: 'session_update' | 'session_exit' | 'session_archived' }>;
 export type WorktreeEvent = Extract<ServerMessage, { type: 'repo_update' | 'repo_removed' | 'worktree_update' | 'worktree_removed' }>;
 export type GroupEvent = Extract<ServerMessage, { type: 'group_update' | 'group_removed' }>;
 /** P7 — signal ciblé « le détail (changes/diff) d'un worktree regardé a changé ». */
@@ -470,7 +470,8 @@ export class WsClient {
         return;
       }
       case 'session_update':
-      case 'session_exit': {
+      case 'session_exit':
+      case 'session_archived': {
         for (const cb of this.sessionListeners) cb(msg);
         return;
       }