feat: v4.0 - 添加 Skills 管理功能

- 新增 Skill 模型，支持手动添加和 GitHub 抓取
- 后台添加 Skills 管理界面
- 前台添加 /skills 列表页和 /skills/<slug> 详情页
- 首页增加 Skills Tab 入口
- 添加数据库迁移脚本

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models.py

@@ -311,3 +311,51 @@ class Collection(db.Model):
     def __repr__(self):
         return f'<Collection user={self.user_id} site={self.site_id}>'
 
+
+class Skill(db.Model):
+    """Skills 模型"""
+    __tablename__ = 'skills'
+
+    id = db.Column(db.Integer, primary_key=True)
+    name = db.Column(db.String(100), nullable=False, comment='Skill名称')
+    slug = db.Column(db.String(100), unique=True, nullable=False, comment='URL别名')
+    description = db.Column(db.Text, comment='详细介绍')
+    short_desc = db.Column(db.String(200), comment='简短描述')
+    logo = db.Column(db.String(500), comment='Logo图片路径')
+    github_url = db.Column(db.String(500), comment='GitHub 链接')
+    source_type = db.Column(db.String(20), default='manual', comment='来源类型: manual/github')
+    source_repo = db.Column(db.String(200), comment='来源仓库')
+    usage = db.Column(db.Text, comment='使用方法')
+    examples = db.Column(db.Text, comment='使用示例')
+    is_active = db.Column(db.Boolean, default=True, comment='是否启用')
+    is_featured = db.Column(db.Boolean, default=False, comment='是否推荐')
+    view_count = db.Column(db.Integer, default=0, comment='浏览次数')
+    sort_order = db.Column(db.Integer, default=0, comment='排序权重')
+    created_at = db.Column(db.DateTime, default=datetime.now, comment='创建时间')
+    updated_at = db.Column(db.DateTime, default=datetime.now, onupdate=datetime.now, comment='更新时间')
+
+    def __repr__(self):
+        return f'<Skill {self.name}>'
+
+    def to_dict(self):
+        """转换为字典"""
+        return {
+            'id': self.id,
+            'name': self.name,
+            'slug': self.slug,
+            'description': self.description,
+            'short_desc': self.short_desc,
+            'logo': self.logo,
+            'github_url': self.github_url,
+            'source_type': self.source_type,
+            'source_repo': self.source_repo,
+            'usage': self.usage,
+            'examples': self.examples,
+            'is_active': self.is_active,
+            'is_featured': self.is_featured,
+            'view_count': self.view_count,
+            'sort_order': self.sort_order,
+            'created_at': self.created_at.strftime('%Y-%m-%d %H:%M:%S') if self.created_at else None,
+            'updated_at': self.updated_at.strftime('%Y-%m-%d %H:%M:%S') if self.updated_at else None
+        }
+