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/post_ai_service.go

@@ -2,10 +2,11 @@ package service
 
 import (
 	"context"
-	"log"
 	"strings"
 
 	"carrot_bbs/internal/pkg/openai"
+
+	"go.uber.org/zap"
 )
 
 // PostModerationRejectedError 帖子审核拒绝错误
@@ -73,7 +74,9 @@ func (s *PostAIService) ModeratePost(ctx context.Context, title, content string,
 		if s.openAIClient.Config().StrictModeration {
 			return err
 		}
-		log.Printf("[WARN] AI moderation failed, fallback allow: %v", err)
+		zap.L().Warn("AI moderation failed, fallback allow",
+			zap.Error(err),
+		)
 		return nil
 	}
 	if !approved {
@@ -93,7 +96,9 @@ func (s *PostAIService) ModerateComment(ctx context.Context, content string, ima
 		if s.openAIClient.Config().StrictModeration {
 			return err
 		}
-		log.Printf("[WARN] AI comment moderation failed, fallback allow: %v", err)
+		zap.L().Warn("AI comment moderation failed, fallback allow",
+			zap.Error(err),
+		)
 		return nil
 	}
 	if !approved {