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>

backend/app/models/bulletin.py

@@ -0,0 +1,59 @@
+"""
+Bulletin Board Models - 게시판 모델
+"""
+from sqlalchemy import Column, Integer, String, Text, Boolean, DateTime, ForeignKey
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+
+from ..database import Base
+
+
+class BoardCategory(Base):
+    """게시판 카테고리"""
+    __tablename__ = "board_categories"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String(100), nullable=False)  # 카테고리명 (한국어)
+    name_en = Column(String(100), nullable=True)  # 영어
+    name_mn = Column(String(100), nullable=True)  # 몽골어
+    name_ru = Column(String(100), nullable=True)  # 러시아어
+    slug = Column(String(50), unique=True, nullable=False)  # URL용 슬러그
+    description = Column(String(255), nullable=True)
+    sort_order = Column(Integer, default=0)
+    is_active = Column(Boolean, default=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
+
+    # Relationships
+    posts = relationship("BoardPost", back_populates="category")
+
+
+class BoardPost(Base):
+    """게시판 글"""
+    __tablename__ = "board_posts"
+
+    id = Column(Integer, primary_key=True, index=True)
+    title = Column(String(255), nullable=False)
+    content = Column(Text, nullable=False)
+
+    # Category
+    category_id = Column(Integer, ForeignKey("board_categories.id"), nullable=False)
+
+    # Author
+    author_id = Column(Integer, ForeignKey("users.id"), nullable=False)
+
+    # Status
+    is_notice = Column(Boolean, default=False)  # 공지사항 여부 (관리자만 설정 가능)
+    is_pinned = Column(Boolean, default=False)  # 상단 고정 여부
+    is_published = Column(Boolean, default=True)  # 게시 여부
+
+    # Stats
+    view_count = Column(Integer, default=0)
+
+    # Timestamps
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
+
+    # Relationships
+    category = relationship("BoardCategory", back_populates="posts")
+    author = relationship("User", foreign_keys=[author_id])