feat: 为 PostgreSQL store 层添加 trace 日志

为 4 个 PostgreSQL repository 添加 trace-aware 日志：
- session_pg.go: Save/Find/Update/Delete 操作日志
- user_pg.go: 用户 CRUD 和 refresh token 管理日志
- message_pg.go: 消息存储和查询日志
- user_scenario_repository.go: 自定义情景 CRUD 日志

日志策略：
- Error: 数据库操作失败
- Debug: 操作成功（避免 Info 级别噪音）
- NotFound (ErrNoRows) 不记录错误日志

backend/internal/store/message_pg.go

@@ -8,6 +8,7 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"github.com/hhs/camtalk/internal/models"
+	"github.com/hhs/camtalk/internal/trace"
 )
 
 // PgMessageRepository 基于 PostgreSQL 的 MessageRepository 实现。
@@ -21,14 +22,24 @@ func NewPgMessageRepository(pool *pgxpool.Pool) *PgMessageRepository {
 }
 
 func (r *PgMessageRepository) SaveMessage(ctx context.Context, sessionID string, msg models.Message, tokensUsed int) error {
+	log := trace.FromContext(ctx)
+
 	_, err := r.pool.Exec(ctx,
 		`INSERT INTO messages (session_id, role, content, tokens_used) VALUES ($1, $2, $3, $4)`,
 		sessionID, msg.Role, msg.Content, tokensUsed,
 	)
-	return err
+	if err != nil {
+		log.Errorw("save message failed", "session_id", sessionID, "role", msg.Role, "error", err)
+		return err
+	}
+
+	log.Debugw("message saved", "session_id", sessionID, "role", msg.Role, "tokens_used", tokensUsed)
+	return nil
 }
 
 func (r *PgMessageRepository) GetMessages(ctx context.Context, sessionID string, limit int, beforeID int64) ([]StoredMessage, error) {
+	log := trace.FromContext(ctx)
+
 	if limit <= 0 {
 		limit = 50
 	}
@@ -56,6 +67,7 @@ func (r *PgMessageRepository) GetMessages(ctx context.Context, sessionID string,
 		)
 	}
 	if err != nil {
+		log.Errorw("get messages failed", "session_id", sessionID, "error", err)
 		return nil, err
 	}
 
@@ -64,12 +76,16 @@ func (r *PgMessageRepository) GetMessages(ctx context.Context, sessionID string,
 		rows[i], rows[j] = rows[j], rows[i]
 	}
 
+	log.Debugw("messages retrieved", "session_id", sessionID, "count", len(rows))
 	return rows, nil
 }
 
 func (r *PgMessageRepository) queryMessages(ctx context.Context, query string, args ...any) ([]StoredMessage, error) {
+	log := trace.FromContext(ctx)
+
 	pgxRows, err := r.pool.Query(ctx, query, args...)
 	if err != nil {
+		log.Errorw("query messages failed", "error", err)
 		return nil, err
 	}
 	defer pgxRows.Close()
@@ -78,17 +94,21 @@ func (r *PgMessageRepository) queryMessages(ctx context.Context, query string, a
 	for pgxRows.Next() {
 		var m StoredMessage
 		if err := pgxRows.Scan(&m.ID, &m.SessionID, &m.Role, &m.Content, &m.TokensUsed, &m.CreatedAt); err != nil {
+			log.Errorw("scan message row failed", "error", err)
 			return nil, err
 		}
 		messages = append(messages, m)
 	}
 	if err := pgxRows.Err(); err != nil {
+		log.Errorw("iterate message rows failed", "error", err)
 		return nil, err
 	}
 	return messages, nil
 }
 
 func (r *PgMessageRepository) GetLastMessage(ctx context.Context, sessionID string) (*StoredMessage, error) {
+	log := trace.FromContext(ctx)
+
 	var m StoredMessage
 	err := r.pool.QueryRow(ctx,
 		`SELECT id, session_id, role, content, tokens_used, created_at
@@ -102,24 +122,34 @@ func (r *PgMessageRepository) GetLastMessage(ctx context.Context, sessionID stri
 		return nil, ErrMessageNotFound
 	}
 	if err != nil {
+		log.Errorw("get last message failed", "session_id", sessionID, "error", err)
 		return nil, err
 	}
+
+	log.Debugw("last message retrieved", "session_id", sessionID, "message_id", m.ID)
 	return &m, nil
 }
 
 func (r *PgMessageRepository) GetMessageCount(ctx context.Context, sessionID string) (int, error) {
+	log := trace.FromContext(ctx)
+
 	var count int
 	err := r.pool.QueryRow(ctx,
 		`SELECT COUNT(*) FROM messages WHERE session_id = $1`,
 		sessionID,
 	).Scan(&count)
 	if err != nil {
+		log.Errorw("get message count failed", "session_id", sessionID, "error", err)
 		return 0, err
 	}
+
+	log.Debugw("message count retrieved", "session_id", sessionID, "count", count)
 	return count, nil
 }
 
 func (r *PgMessageRepository) GetSessionMessageStats(ctx context.Context, sessionIDs []string) (map[string]SessionMessageStats, error) {
+	log := trace.FromContext(ctx)
+
 	if len(sessionIDs) == 0 {
 		return map[string]SessionMessageStats{}, nil
 	}
@@ -143,6 +173,7 @@ func (r *PgMessageRepository) GetSessionMessageStats(ctx context.Context, sessio
 		sessionIDs,
 	)
 	if err != nil {
+		log.Errorw("get session message stats failed", "session_count", len(sessionIDs), "error", err)
 		return nil, err
 	}
 	defer rows.Close()
@@ -152,12 +183,16 @@ func (r *PgMessageRepository) GetSessionMessageStats(ctx context.Context, sessio
 		var sid string
 		var stats SessionMessageStats
 		if err := rows.Scan(&sid, &stats.MessageCount, &stats.LastMessage); err != nil {
+			log.Errorw("scan message stats row failed", "error", err)
 			return nil, err
 		}
 		result[sid] = stats
 	}
 	if err := rows.Err(); err != nil {
+		log.Errorw("iterate message stats rows failed", "error", err)
 		return nil, err
 	}
+
+	log.Debugw("session message stats retrieved", "session_count", len(sessionIDs), "result_count", len(result))
 	return result, nil
 }