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/user_scenario_repository.go

@@ -10,6 +10,7 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"github.com/hhs/camtalk/internal/models"
+	"github.com/hhs/camtalk/internal/trace"
 )
 
 // UserScenarioRepository 用户自建情景仓储接口。
@@ -35,6 +36,8 @@ func NewPostgresUserScenarioRepo(pool *pgxpool.Pool) UserScenarioRepository {
 
 // Create 创建用户情景。
 func (r *PostgresUserScenarioRepo) Create(ctx context.Context, scenario *models.UserScenario) error {
+	log := trace.FromContext(ctx)
+
 	query := `
 		INSERT INTO user_scenarios (id, user_id, name, icon, description, prompt, greeting, language, created_at, updated_at)
 		VALUES ($1, $2, $3, $4, NULLIF($5, ''), $6, NULLIF($7, ''), $8, $9, $10)
@@ -69,13 +72,18 @@ func (r *PostgresUserScenarioRepo) Create(ctx context.Context, scenario *models.
 	).Scan(&scenario.ID, &scenario.CreatedAt, &scenario.UpdatedAt)
 
 	if err != nil {
+		log.Errorw("create user scenario failed", "user_id", scenario.UserID, "name", scenario.Name, "error", err)
 		return fmt.Errorf("create user scenario: %w", err)
 	}
+
+	log.Debugw("user scenario created", "scenario_id", scenario.ID, "user_id", scenario.UserID, "name", scenario.Name)
 	return nil
 }
 
 // FindByID 根据 ID 查找情景。
 func (r *PostgresUserScenarioRepo) FindByID(ctx context.Context, id string) (*models.UserScenario, error) {
+	log := trace.FromContext(ctx)
+
 	query := `
 		SELECT id, user_id, name, icon, description, prompt, greeting, language, created_at, updated_at
 		FROM user_scenarios
@@ -100,13 +108,18 @@ func (r *PostgresUserScenarioRepo) FindByID(ctx context.Context, id string) (*mo
 		return nil, fmt.Errorf("user scenario not found: %s", id)
 	}
 	if err != nil {
+		log.Errorw("find user scenario failed", "scenario_id", id, "error", err)
 		return nil, fmt.Errorf("find user scenario: %w", err)
 	}
+
+	log.Debugw("user scenario found", "scenario_id", id)
 	return &scenario, nil
 }
 
 // FindByIDAndUserID 根据 ID 和用户 ID 查找情景（权限校验）。
 func (r *PostgresUserScenarioRepo) FindByIDAndUserID(ctx context.Context, id, userID string) (*models.UserScenario, error) {
+	log := trace.FromContext(ctx)
+
 	query := `
 		SELECT id, user_id, name, icon, description, prompt, greeting, language, created_at, updated_at
 		FROM user_scenarios
@@ -131,13 +144,18 @@ func (r *PostgresUserScenarioRepo) FindByIDAndUserID(ctx context.Context, id, us
 		return nil, fmt.Errorf("user scenario not found or no permission")
 	}
 	if err != nil {
+		log.Errorw("find user scenario by id and user failed", "scenario_id", id, "user_id", userID, "error", err)
 		return nil, fmt.Errorf("find user scenario: %w", err)
 	}
+
+	log.Debugw("user scenario found by id and user", "scenario_id", id, "user_id", userID)
 	return &scenario, nil
 }
 
 // FindByUserID 查找用户的所有情景。
 func (r *PostgresUserScenarioRepo) FindByUserID(ctx context.Context, userID string) ([]*models.UserScenario, error) {
+	log := trace.FromContext(ctx)
+
 	query := `
 		SELECT id, user_id, name, icon, description, prompt, greeting, language, created_at, updated_at
 		FROM user_scenarios
@@ -147,6 +165,7 @@ func (r *PostgresUserScenarioRepo) FindByUserID(ctx context.Context, userID stri
 
 	rows, err := r.pool.Query(ctx, query, userID)
 	if err != nil {
+		log.Errorw("find user scenarios failed", "user_id", userID, "error", err)
 		return nil, fmt.Errorf("find user scenarios: %w", err)
 	}
 	defer rows.Close()
@@ -167,19 +186,25 @@ func (r *PostgresUserScenarioRepo) FindByUserID(ctx context.Context, userID stri
 			&s.UpdatedAt,
 		)
 		if err != nil {
+			log.Errorw("scan user scenario row failed", "user_id", userID, "error", err)
 			return nil, fmt.Errorf("scan user scenario: %w", err)
 		}
 		scenarios = append(scenarios, &s)
 	}
 
 	if err = rows.Err(); err != nil {
+		log.Errorw("iterate user scenarios failed", "user_id", userID, "error", err)
 		return nil, fmt.Errorf("iterate user scenarios: %w", err)
 	}
+
+	log.Debugw("user scenarios found", "user_id", userID, "count", len(scenarios))
 	return scenarios, nil
 }
 
 // Update 更新用户情景。
 func (r *PostgresUserScenarioRepo) Update(ctx context.Context, scenario *models.UserScenario) error {
+	log := trace.FromContext(ctx)
+
 	query := `
 		UPDATE user_scenarios
 		SET name = $1, icon = $2, description = $3, prompt = $4, greeting = $5, language = $6, updated_at = $7
@@ -205,34 +230,47 @@ func (r *PostgresUserScenarioRepo) Update(ctx context.Context, scenario *models.
 		return fmt.Errorf("user scenario not found or no permission")
 	}
 	if err != nil {
+		log.Errorw("update user scenario failed", "scenario_id", scenario.ID, "user_id", scenario.UserID, "error", err)
 		return fmt.Errorf("update user scenario: %w", err)
 	}
+
+	log.Debugw("user scenario updated", "scenario_id", scenario.ID, "user_id", scenario.UserID)
 	return nil
 }
 
 // Delete 删除用户情景。
 func (r *PostgresUserScenarioRepo) Delete(ctx context.Context, id string) error {
+	log := trace.FromContext(ctx)
+
 	query := `DELETE FROM user_scenarios WHERE id = $1`
 
 	result, err := r.pool.Exec(ctx, query, id)
 	if err != nil {
+		log.Errorw("delete user scenario failed", "scenario_id", id, "error", err)
 		return fmt.Errorf("delete user scenario: %w", err)
 	}
 
 	if result.RowsAffected() == 0 {
 		return fmt.Errorf("user scenario not found")
 	}
+
+	log.Debugw("user scenario deleted", "scenario_id", id)
 	return nil
 }
 
 // CountByUserID 统计用户的情景数量。
 func (r *PostgresUserScenarioRepo) CountByUserID(ctx context.Context, userID string) (int, error) {
+	log := trace.FromContext(ctx)
+
 	query := `SELECT COUNT(*) FROM user_scenarios WHERE user_id = $1`
 
 	var count int
 	err := r.pool.QueryRow(ctx, query, userID).Scan(&count)
 	if err != nil {
+		log.Errorw("count user scenarios failed", "user_id", userID, "error", err)
 		return 0, fmt.Errorf("count user scenarios: %w", err)
 	}
+
+	log.Debugw("user scenarios counted", "user_id", userID, "count", count)
 	return count, nil
 }