feat(admin): web admin panel — user CRUD, role assignment, system health (#150)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/admin/admin.guard.ts

@@ -0,0 +1,44 @@
+import {
+  CanActivate,
+  ExecutionContext,
+  ForbiddenException,
+  Inject,
+  Injectable,
+  UnauthorizedException,
+} from '@nestjs/common';
+import { fromNodeHeaders } from 'better-auth/node';
+import type { Auth } from '@mosaic/auth';
+import type { FastifyRequest } from 'fastify';
+import { AUTH } from '../auth/auth.tokens.js';
+
+interface UserWithRole {
+  id: string;
+  role?: string;
+}
+
+@Injectable()
+export class AdminGuard implements CanActivate {
+  constructor(@Inject(AUTH) private readonly auth: Auth) {}
+
+  async canActivate(context: ExecutionContext): Promise<boolean> {
+    const request = context.switchToHttp().getRequest<FastifyRequest>();
+    const headers = fromNodeHeaders(request.raw.headers);
+
+    const result = await this.auth.api.getSession({ headers });
+
+    if (!result) {
+      throw new UnauthorizedException('Invalid or expired session');
+    }
+
+    const user = result.user as UserWithRole;
+
+    if (user.role !== 'admin') {
+      throw new ForbiddenException('Admin access required');
+    }
+
+    (request as FastifyRequest & { user: unknown; session: unknown }).user = result.user;
+    (request as FastifyRequest & { user: unknown; session: unknown }).session = result.session;
+
+    return true;
+  }
+}