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/message_service.go

@@ -2,13 +2,13 @@ package service
 
 import (
 	"context"
-	"log"
 	"time"
 
 	"carrot_bbs/internal/cache"
 	"carrot_bbs/internal/model"
 	"carrot_bbs/internal/repository"
 
+	"go.uber.org/zap"
 	"gorm.io/gorm"
 )
 
@@ -94,7 +94,12 @@ func (s *MessageService) SendMessage(ctx context.Context, senderID, receiverID s
 	// 异步写入缓存
 	go func() {
 		if err := s.cacheMessage(context.Background(), conv.ID, msg); err != nil {
-			log.Printf("[MessageService] async cache message failed, convID=%s, msgID=%s, err=%v", conv.ID, msg.ID, err)
+			zap.L().Warn("async cache message failed",
+				zap.String("component", "MessageService"),
+				zap.String("convID", conv.ID),
+				zap.String("msgID", msg.ID),
+				zap.Error(err),
+			)
 		}
 	}()