feat(server): integrate Better Auth, access control, and deployment mode startup

Wire up Better Auth for session-based authentication. Add actor middleware
that resolves local_trusted mode to an implicit board actor and authenticated
mode to Better Auth sessions. Add access service with membership, permission,
invite, and join-request management. Register access routes for member/invite/
join-request CRUD. Update health endpoint to report deployment mode and
bootstrap status. Enforce tasks:assign and agents:create permissions in issue
and agent routes. Add deployment mode validation at startup with guardrails
(loopback-only for local_trusted, auth config required for authenticated).

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

server/src/routes/index.ts

@@ -11,3 +11,4 @@ export { activityRoutes } from "./activity.js";
 export { dashboardRoutes } from "./dashboard.js";
 export { sidebarBadgeRoutes } from "./sidebar-badges.js";
 export { llmRoutes } from "./llms.js";
+export { accessRoutes } from "./access.js";