feat(group): refactor group invite workflow for improved user experience

- Simplify InviteMembers to always send invite card directly to invitee
- Modify RespondInvite to check group join type (anyone vs need approval) before adding member
- Update SetGroupAddRequest to properly handle invite flow with invitee agreement and admin approval stages
- Add case-insensitive search using LOWER() in user repository
- Add random offset (0-50) for first page of recommended posts
- Update dependencies: remove gorilla/websocket, promote gomail to direct dependency

internal/service/post_service.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"math/rand"
 	"strings"
 	"time"
 
@@ -255,7 +256,7 @@ func (s *PostService) List(ctx context.Context, page, pageSize int, userID strin
 	}
 	cacheKey := cache.PostListKey("latest", visibilityUserKey, page, pageSize)
 
-	result, err := cache.GetOrLoadTyped[*PostListResult](
+	result, err := cache.GetOrLoadTyped(
 		s.cache,
 		cacheKey,
 		postListTTL,
@@ -595,8 +596,14 @@ func (s *PostService) GetRecommendedPosts(ctx context.Context, userID string, pa
 		return s.GetHotPosts(ctx, page, pageSize)
 	}
 
-	// 计算偏移量
-	offset := (page - 1) * pageSize
+	// 计算偏移量：第一页使用随机偏移量，增加推荐多样性
+	var offset int
+	if page == 1 {
+		// 第一页随机偏移 0-50，让每次刷新看到不同的推荐内容
+		offset = rand.Intn(51)
+	} else {
+		offset = (page - 1) * pageSize
+	}
 
 	// 从Gorse获取推荐列表
 	// 多取1条用于判断是否还有下一页