feat(multiuser): phase 5a, admin user-management API

- routes/admin-routes.ts: GET/POST /api/admin/users, PATCH/DELETE
  /api/admin/users/:username, reset-password, logout. Multi-user only (404
  otherwise), requireAdmin, last-admin invariants, one-time-password on create /
  reset (returned once + mustChangePassword), disable/reset/delete revoke cookie
  sessions, delete kills the user's live sessions first (normal teardown) and can
  delete their space (guarded). Per-user stats (live/active sessions, case count).
- web/admin-audit.ts: append-only ~/.codeman/admin-audit.jsonl (timestamp, acting
  admin, action, target, IP) for every user-management action.
- SSE admin:usersChanged + auth:passwordChangeRequired (sse-events.ts + constants.js).

fix(user-store): serialize users.json read-modify-write

touchLastLogin fires on every Basic auth (fire-and-forget) and was racing route
writes (create/update), clobbering records — a real corruption bug surfaced by
the admin tests. All mutators now run under a single write lock, and
touchLastLogin is throttled to once/minute per user to bound disk churn.

Tests: test/admin-routes.test.ts (8, live server) + user-store lock verified by
the existing user-store suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/routes/index.ts

@@ -20,4 +20,5 @@ export { registerOrchestratorRoutes } from './orchestrator-routes.js';
 export { registerClipboardRoutes } from './clipboard-routes.js';
 export { registerSearchRoutes } from './search-routes.js';
 export { registerMeRoutes } from './me-routes.js';
+export { registerAdminRoutes } from './admin-routes.js';
 export { registerWsRoutes } from './ws-routes.js';