feat: Add bulletin board system

- Add BoardCategory and BoardPost models with multi-language support
- Add bulletin API endpoints (CRUD, notice toggle, pin toggle)
- Add board_enabled setting to control menu visibility
- Create frontend board pages (list, detail, write, edit)
- Create admin board management and category management pages
- Update Header.tsx with conditional Board menu between Inquiry and Contact Us
- Update admin settings with board_enabled toggle
- Add Board menu to admin sidebar

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -1765,4 +1765,174 @@ export const snsShareApi = {
   },
 };
 
+// Board (Bulletin) API Types
+export interface BoardCategory {
+  id: number;
+  name: string;
+  name_en?: string;
+  name_mn?: string;
+  name_ru?: string;
+  slug: string;
+  description?: string;
+  sort_order: number;
+  is_active: boolean;
+  created_at: string;
+  updated_at?: string;
+  post_count: number;
+}
+
+export interface BoardPostListItem {
+  id: number;
+  title: string;
+  category_id: number;
+  category_name?: string;
+  author_id: number;
+  author_name?: string;
+  is_notice: boolean;
+  is_pinned: boolean;
+  view_count: number;
+  created_at: string;
+}
+
+export interface BoardPost {
+  id: number;
+  title: string;
+  content: string;
+  category_id: number;
+  category?: BoardCategory;
+  author_id: number;
+  author?: {
+    id: number;
+    name?: string;
+    email: string;
+    is_admin: boolean;
+  };
+  is_notice: boolean;
+  is_pinned: boolean;
+  is_published: boolean;
+  view_count: number;
+  created_at: string;
+  updated_at?: string;
+}
+
+export interface BoardPostListResponse {
+  posts: BoardPostListItem[];
+  notices: BoardPostListItem[];
+  total: number;
+  page: number;
+  page_size: number;
+  total_pages: number;
+}
+
+export interface BoardCategoryListResponse {
+  categories: BoardCategory[];
+  total: number;
+}
+
+// Board API
+export const boardApi = {
+  // Categories
+  getCategories: async (includeInactive: boolean = false): Promise<BoardCategoryListResponse> => {
+    const { data } = await api.get('/board/categories', { params: { include_inactive: includeInactive } });
+    return data;
+  },
+
+  createCategory: async (category: {
+    name: string;
+    name_en?: string;
+    name_mn?: string;
+    name_ru?: string;
+    slug: string;
+    description?: string;
+    sort_order?: number;
+    is_active?: boolean;
+  }): Promise<BoardCategory> => {
+    const { data } = await api.post('/board/categories', category);
+    return data;
+  },
+
+  updateCategory: async (categoryId: number, category: {
+    name?: string;
+    name_en?: string;
+    name_mn?: string;
+    name_ru?: string;
+    slug?: string;
+    description?: string;
+    sort_order?: number;
+    is_active?: boolean;
+  }): Promise<BoardCategory> => {
+    const { data } = await api.put(`/board/categories/${categoryId}`, category);
+    return data;
+  },
+
+  deleteCategory: async (categoryId: number): Promise<void> => {
+    await api.delete(`/board/categories/${categoryId}`);
+  },
+
+  // Posts
+  getPosts: async (params: {
+    page?: number;
+    page_size?: number;
+    category_id?: number;
+    search?: string;
+  }): Promise<BoardPostListResponse> => {
+    const { data } = await api.get('/board/posts', { params });
+    return data;
+  },
+
+  getPost: async (postId: number): Promise<BoardPost> => {
+    const { data } = await api.get(`/board/posts/${postId}`);
+    return data;
+  },
+
+  createPost: async (post: {
+    title: string;
+    content: string;
+    category_id: number;
+    is_notice?: boolean;
+  }): Promise<BoardPost> => {
+    const { data } = await api.post('/board/posts', post);
+    return data;
+  },
+
+  updatePost: async (postId: number, post: {
+    title?: string;
+    content?: string;
+    category_id?: number;
+    is_notice?: boolean;
+    is_pinned?: boolean;
+    is_published?: boolean;
+  }): Promise<BoardPost> => {
+    const { data } = await api.put(`/board/posts/${postId}`, post);
+    return data;
+  },
+
+  deletePost: async (postId: number): Promise<void> => {
+    await api.delete(`/board/posts/${postId}`);
+  },
+
+  // Admin
+  getAdminPosts: async (params: {
+    page?: number;
+    page_size?: number;
+    category_id?: number;
+    is_notice?: boolean;
+    is_published?: boolean;
+    search?: string;
+  }): Promise<BoardPostListResponse> => {
+    const { data } = await api.get('/board/admin/posts', { params });
+    return data;
+  },
+
+  toggleNotice: async (postId: number): Promise<{ is_notice: boolean; is_pinned: boolean }> => {
+    const { data } = await api.put(`/board/admin/posts/${postId}/toggle-notice`);
+    return data;
+  },
+
+  togglePin: async (postId: number): Promise<{ is_pinned: boolean }> => {
+    const { data } = await api.put(`/board/admin/posts/${postId}/toggle-pin`);
+    return data;
+  },
+};
+
 export default api;