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/services/heartbeat.ts

@@ -985,7 +985,7 @@ export function heartbeatService(db: Db) {
             : outcome === "cancelled"
               ? "cancelled"
               : outcome === "failed"
-                ? "adapter_failed"
+                ? (adapterResult.errorCode ?? "adapter_failed")
                 : null,
         exitCode: adapterResult.exitCode,
         signal: adapterResult.signal,