feat(post): implement share recording functionality

- Added RecordShare method in PostHandler to handle sharing posts and increment share count.
- Introduced IncrementShares method in PostRepository to update shares_count for published posts.
- Updated PostService to include RecordShare, ensuring cache invalidation for post details and lists.
- Integrated new share route in the router for handling share requests.

internal/router/router.go

@@ -220,6 +220,8 @@ func (r *Router) setupRoutes() {
 
 			// 浏览量记录（可选认证，允许游客浏览）
 			posts.POST("/:id/view", middleware.OptionalAuth(r.jwtService), r.postHandler.RecordView)
+			// 分享计数（可选认证；仅已发布帖子生效）
+			posts.POST("/:id/share", middleware.OptionalAuth(r.jwtService), r.postHandler.RecordShare)
 
 			// 点赞
 			posts.POST("/:id/like", authMiddleware, r.postHandler.Like)