feat: admin back-office system + AI assistant with action commands

- Admin analytics (recharts charts, overview stats, trend analysis, time range selector)
- Admin permissions (AdminRole/AdminUser models, role CRUD, permission catalog)
- Admin system config (site/AI/member category tabs, per-key save)
- Admin operations (Banner CRUD, push notification send/delete)
- Admin user management (table, search, create/edit, ban/delete)
- Admin layout (custom top bar with branding + sidebar, admin AI assistant)
- Admin login (adminToken localStorage, adminInfo display)
- Admin AI assistant (purple '运营助手', context-aware prompts, action commands)
- Public AI assistant Phase B (action commands: navigate, setModel, startChat, openSkill, setParameter)
- Assistant context mapping + action executor
- Skills API fix: tags parsing fallback (JSON.parse → split)
- Sandbox crash fix: curScene fallback system prompt
- JWT token expiration: access 2h→7d, refresh 7d→30d, admin 8h→7d
- i18n: assistant-related translations (zh/en)
- PM2 ecosystem config for process management
- AI assistant login prompt fix: admin uses getAdminToken(), public uses apiFetch with token refresh

backend/prisma/schema.prisma

@@ -337,16 +337,30 @@ model Subscription {
   @@map("subscriptions")
 }
 
+model AdminRole {
+  id          String   @id @default(cuid())
+  name        String   @unique
+  description String?
+  permissions Json     @default("[]")
+  status      String   @default("ACTIVE")
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+  admins      AdminUser[]
+
+  @@map("admin_roles")
+}
+
 model AdminUser {
-  id           Int       @id @default(autoincrement())
-  username     String    @unique
+  id           Int        @id @default(autoincrement())
+  username     String     @unique
   passwordHash String
   nickname     String?
-  role         String    @default("editor")
-  status       String    @default("ACTIVE")
+  roleId       String?
+  role         AdminRole? @relation(fields: [roleId], references: [id])
+  status       String     @default("ACTIVE")
   lastLoginAt  DateTime?
-  createdAt    DateTime  @default(now())
-  updatedAt    DateTime  @updatedAt
+  createdAt    DateTime   @default(now())
+  updatedAt    DateTime   @updatedAt
 
   @@map("admin_users")
 }
@@ -529,3 +543,48 @@ model Notification {
   @@index([userId, isRead])
   @@map("notifications")
 }
+
+model Banner {
+  id          Int       @id @default(autoincrement())
+  title       String
+  image       String
+  link        String?
+  position    String    @default("home")
+  sortOrder   Int       @default(0)
+  status      String    @default("DRAFT")
+  startAt     DateTime?
+  endAt       DateTime?
+  createdAt   DateTime  @default(now())
+  updatedAt   DateTime  @updatedAt
+
+  @@map("banners")
+}
+
+model SystemNotification {
+  id          Int        @id @default(autoincrement())
+  title       String
+  content     String
+  type        String    @default("system")
+  target      String    @default("all")
+  targetIds   String?
+  status      String    @default("DRAFT")
+  scheduledAt DateTime?
+  sentAt      DateTime?
+  createdAt   DateTime  @default(now())
+  updatedAt   DateTime  @updatedAt
+
+  @@map("system_notifications")
+}
+
+model SystemConfig {
+  id          String   @id @default(cuid())
+  category    String
+  key         String   @unique
+  value       String
+  description String?
+  status      String   @default("ACTIVE")
+  createdAt   DateTime @default(now())
+  updatedAt   DateTime @updatedAt
+
+  @@map("system_configs")
+}