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.dto.ts

@@ -0,0 +1,56 @@
+export interface UserDto {
+  id: string;
+  name: string;
+  email: string;
+  role: string;
+  banned: boolean;
+  banReason: string | null;
+  createdAt: string;
+  updatedAt: string;
+}
+
+export interface UserListDto {
+  users: UserDto[];
+  total: number;
+}
+
+export interface CreateUserDto {
+  name: string;
+  email: string;
+  password: string;
+  role?: string;
+}
+
+export interface UpdateUserRoleDto {
+  role: string;
+}
+
+export interface BanUserDto {
+  reason?: string;
+}
+
+export interface HealthStatusDto {
+  status: 'ok' | 'degraded' | 'error';
+  database: ServiceStatusDto;
+  cache: ServiceStatusDto;
+  agentPool: AgentPoolStatusDto;
+  providers: ProviderStatusDto[];
+  checkedAt: string;
+}
+
+export interface ServiceStatusDto {
+  status: 'ok' | 'error';
+  latencyMs?: number;
+  error?: string;
+}
+
+export interface AgentPoolStatusDto {
+  activeSessions: number;
+}
+
+export interface ProviderStatusDto {
+  id: string;
+  name: string;
+  available: boolean;
+  modelCount: number;
+}