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/session_pg.go

@@ -6,6 +6,8 @@ import (
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/hhs/camtalk/internal/trace"
 )
 
 // PgSessionRepository 基于 PostgreSQL 的 SessionRepository 实现。
@@ -19,6 +21,8 @@ func NewPgSessionRepository(pool *pgxpool.Pool) *PgSessionRepository {
 }
 
 func (r *PgSessionRepository) Save(ctx context.Context, s SessionRecord) error {
+	log := trace.FromContext(ctx)
+
 	_, err := r.pool.Exec(ctx,
 		`INSERT INTO sessions (id, user_id, title, config, created_at, updated_at)
 		 VALUES ($1, $2, $3, $4, $5, $6)
@@ -28,10 +32,18 @@ func (r *PgSessionRepository) Save(ctx context.Context, s SessionRecord) error {
 		   updated_at = EXCLUDED.updated_at`,
 		s.ID, s.UserID, s.Title, s.Config, s.CreatedAt, s.UpdatedAt,
 	)
-	return err
+	if err != nil {
+		log.Errorw("save session failed", "session_id", s.ID, "error", err)
+		return err
+	}
+
+	log.Debugw("session saved", "session_id", s.ID, "user_id", s.UserID)
+	return nil
 }
 
 func (r *PgSessionRepository) FindByID(ctx context.Context, id string) (*SessionRecord, error) {
+	log := trace.FromContext(ctx)
+
 	var s SessionRecord
 	err := r.pool.QueryRow(ctx,
 		`SELECT id, user_id, title, config, created_at, updated_at
@@ -41,12 +53,17 @@ func (r *PgSessionRepository) FindByID(ctx context.Context, id string) (*Session
 		return nil, ErrSessionNotFound
 	}
 	if err != nil {
+		log.Errorw("find session failed", "session_id", id, "error", err)
 		return nil, err
 	}
+
+	log.Debugw("session found", "session_id", id)
 	return &s, nil
 }
 
 func (r *PgSessionRepository) FindByUser(ctx context.Context, userID string, page, size int) ([]SessionRecord, int, error) {
+	log := trace.FromContext(ctx)
+
 	if page <= 0 {
 		page = 1
 	}
@@ -60,6 +77,7 @@ func (r *PgSessionRepository) FindByUser(ctx context.Context, userID string, pag
 	if err := r.pool.QueryRow(ctx,
 		`SELECT COUNT(*) FROM sessions WHERE user_id = $1`, userID,
 	).Scan(&total); err != nil {
+		log.Errorw("count user sessions failed", "user_id", userID, "error", err)
 		return nil, 0, err
 	}
 
@@ -73,6 +91,7 @@ func (r *PgSessionRepository) FindByUser(ctx context.Context, userID string, pag
 		userID, size, offset,
 	)
 	if err != nil {
+		log.Errorw("find user sessions failed", "user_id", userID, "error", err)
 		return nil, 0, err
 	}
 	defer rows.Close()
@@ -81,66 +100,90 @@ func (r *PgSessionRepository) FindByUser(ctx context.Context, userID string, pag
 	for rows.Next() {
 		var s SessionRecord
 		if err := rows.Scan(&s.ID, &s.UserID, &s.Title, &s.Config, &s.CreatedAt, &s.UpdatedAt); err != nil {
+			log.Errorw("scan session row failed", "user_id", userID, "error", err)
 			return nil, 0, err
 		}
 		list = append(list, s)
 	}
 	if err := rows.Err(); err != nil {
+		log.Errorw("iterate session rows failed", "user_id", userID, "error", err)
 		return nil, 0, err
 	}
+
+	log.Debugw("user sessions found", "user_id", userID, "count", len(list), "total", total)
 	return list, total, nil
 }
 
 func (r *PgSessionRepository) UpdateTitle(ctx context.Context, id string, title string) error {
+	log := trace.FromContext(ctx)
+
 	tag, err := r.pool.Exec(ctx,
 		`UPDATE sessions SET title = $2, updated_at = NOW() WHERE id = $1`,
 		id, title,
 	)
 	if err != nil {
+		log.Errorw("update session title failed", "session_id", id, "error", err)
 		return err
 	}
 	if tag.RowsAffected() == 0 {
 		return ErrSessionNotFound
 	}
+
+	log.Debugw("session title updated", "session_id", id)
 	return nil
 }
 
 func (r *PgSessionRepository) UpdateConfig(ctx context.Context, id string, configJSON []byte) error {
+	log := trace.FromContext(ctx)
+
 	tag, err := r.pool.Exec(ctx,
 		`UPDATE sessions SET config = $2, updated_at = NOW() WHERE id = $1`,
 		id, configJSON,
 	)
 	if err != nil {
+		log.Errorw("update session config failed", "session_id", id, "error", err)
 		return err
 	}
 	if tag.RowsAffected() == 0 {
 		return ErrSessionNotFound
 	}
+
+	log.Debugw("session config updated", "session_id", id)
 	return nil
 }
 
 func (r *PgSessionRepository) Touch(ctx context.Context, id string) error {
+	log := trace.FromContext(ctx)
+
 	tag, err := r.pool.Exec(ctx,
 		`UPDATE sessions SET updated_at = NOW() WHERE id = $1`, id,
 	)
 	if err != nil {
+		log.Errorw("touch session failed", "session_id", id, "error", err)
 		return err
 	}
 	if tag.RowsAffected() == 0 {
 		return ErrSessionNotFound
 	}
+
+	log.Debugw("session touched", "session_id", id)
 	return nil
 }
 
 func (r *PgSessionRepository) Delete(ctx context.Context, id string) error {
+	log := trace.FromContext(ctx)
+
 	tag, err := r.pool.Exec(ctx,
 		`DELETE FROM sessions WHERE id = $1`, id,
 	)
 	if err != nil {
+		log.Errorw("delete session failed", "session_id", id, "error", err)
 		return err
 	}
 	if tag.RowsAffected() == 0 {
 		return ErrSessionNotFound
 	}
+
+	log.Debugw("session deleted", "session_id", id)
 	return nil
 }