refactor: replace standard log with zap logger and optimize code patterns

- Migrate all log.Printf/log.Println calls to structured zap logging
- Use cmp.Or for cleaner default value handling
- Replace manual loops with slices.ContainsFunc/IndexFunc
- Add batch query methods (IsLikedBatch, IsFavoritedBatch) to solve N+1 problem
- Update JSON tags from omitempty to omitzero for numeric fields
- Use errgroup-style wg.Go for worker goroutines
- Remove duplicate casbin/v2 dependency (keeping v3)
- Add plans/ to gitignore

internal/service/chat_service.go

@@ -4,7 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"log"
+	"slices"
 	"time"
 
 	"carrot_bbs/internal/cache"
@@ -13,6 +13,7 @@ import (
 	"carrot_bbs/internal/pkg/sse"
 	"carrot_bbs/internal/repository"
 
+	"go.uber.org/zap"
 	"gorm.io/gorm"
 )
 
@@ -298,7 +299,12 @@ func (s *chatServiceImpl) SendMessage(ctx context.Context, senderID string, conv
 	// 异步写入缓存
 	go func() {
 		if err := s.cacheMessage(context.Background(), conversationID, message); err != nil {
-			log.Printf("[ChatService] async cache message failed, convID=%s, msgID=%s, err=%v", conversationID, message.ID, err)
+			zap.L().Warn("async cache message failed",
+				zap.String("component", "ChatService"),
+				zap.String("convID", conversationID),
+				zap.String("msgID", message.ID),
+				zap.Error(err),
+			)
 		}
 	}()
 
@@ -379,12 +385,9 @@ func (s *chatServiceImpl) cacheMessage(ctx context.Context, convID string, msg *
 }
 
 func containsImageSegment(segments model.MessageSegments) bool {
-	for _, seg := range segments {
-		if seg.Type == string(model.ContentTypeImage) || seg.Type == "image" {
-			return true
-		}
-	}
-	return false
+	return slices.ContainsFunc(segments, func(seg model.MessageSegment) bool {
+		return seg.Type == string(model.ContentTypeImage) || seg.Type == "image"
+	})
 }
 
 // GetMessages 获取消息历史（分页，带缓存）
@@ -718,7 +721,12 @@ func (s *chatServiceImpl) SaveMessage(ctx context.Context, senderID string, conv
 	// 异步写入缓存
 	go func() {
 		if err := s.cacheMessage(context.Background(), conversationID, message); err != nil {
-			log.Printf("[ChatService] async cache message failed, convID=%s, msgID=%s, err=%v", conversationID, message.ID, err)
+			zap.L().Warn("async cache message failed",
+				zap.String("component", "ChatService"),
+				zap.String("convID", conversationID),
+				zap.String("msgID", message.ID),
+				zap.Error(err),
+			)
 		}
 	}()