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_pg.go

@@ -7,6 +7,8 @@ import (
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
+
+	"github.com/hhs/camtalk/internal/trace"
 )
 
 // PgUserRepository 基于 PostgreSQL 的 UserRepository 实现。
@@ -20,18 +22,25 @@ func NewPgUserRepository(pool *pgxpool.Pool) *PgUserRepository {
 }
 
 func (r *PgUserRepository) Create(ctx context.Context, username, passwordHash string) (string, error) {
+	log := trace.FromContext(ctx)
+
 	var id string
 	err := r.pool.QueryRow(ctx,
 		`INSERT INTO users (username, password_hash) VALUES ($1, $2) RETURNING id`,
 		username, passwordHash,
 	).Scan(&id)
 	if err != nil {
+		log.Errorw("create user failed", "username", username, "error", err)
 		return "", err
 	}
+
+	log.Debugw("user created", "user_id", id, "username", username)
 	return id, nil
 }
 
 func (r *PgUserRepository) FindByUsername(ctx context.Context, username string) (*User, error) {
+	log := trace.FromContext(ctx)
+
 	var u User
 	err := r.pool.QueryRow(ctx,
 		`SELECT id, username, password_hash, created_at, updated_at FROM users WHERE username = $1`,
@@ -41,12 +50,17 @@ func (r *PgUserRepository) FindByUsername(ctx context.Context, username string)
 		return nil, ErrUserNotFound
 	}
 	if err != nil {
+		log.Errorw("find user by username failed", "username", username, "error", err)
 		return nil, err
 	}
+
+	log.Debugw("user found by username", "user_id", u.ID, "username", username)
 	return &u, nil
 }
 
 func (r *PgUserRepository) FindByID(ctx context.Context, id string) (*User, error) {
+	log := trace.FromContext(ctx)
+
 	var u User
 	err := r.pool.QueryRow(ctx,
 		`SELECT id, username, password_hash, created_at, updated_at FROM users WHERE id = $1`,
@@ -56,20 +70,33 @@ func (r *PgUserRepository) FindByID(ctx context.Context, id string) (*User, erro
 		return nil, ErrUserNotFound
 	}
 	if err != nil {
+		log.Errorw("find user by id failed", "user_id", id, "error", err)
 		return nil, err
 	}
+
+	log.Debugw("user found by id", "user_id", id)
 	return &u, nil
 }
 
 func (r *PgUserRepository) SaveRefreshToken(ctx context.Context, userID, tokenHash string, expiresAt time.Time) error {
+	log := trace.FromContext(ctx)
+
 	_, err := r.pool.Exec(ctx,
 		`INSERT INTO refresh_tokens (user_id, token_hash, expires_at) VALUES ($1, $2, $3)`,
 		userID, tokenHash, expiresAt,
 	)
-	return err
+	if err != nil {
+		log.Errorw("save refresh token failed", "user_id", userID, "error", err)
+		return err
+	}
+
+	log.Debugw("refresh token saved", "user_id", userID)
+	return nil
 }
 
 func (r *PgUserRepository) FindRefreshToken(ctx context.Context, tokenHash string) (string, error) {
+	log := trace.FromContext(ctx)
+
 	var userID string
 	err := r.pool.QueryRow(ctx,
 		`SELECT user_id FROM refresh_tokens WHERE token_hash = $1 AND expires_at > NOW()`,
@@ -79,23 +106,42 @@ func (r *PgUserRepository) FindRefreshToken(ctx context.Context, tokenHash strin
 		return "", ErrRefreshTokenNotFound
 	}
 	if err != nil {
+		log.Errorw("find refresh token failed", "error", err)
 		return "", err
 	}
+
+	log.Debugw("refresh token found", "user_id", userID)
 	return userID, nil
 }
 
 func (r *PgUserRepository) DeleteRefreshToken(ctx context.Context, tokenHash string) error {
+	log := trace.FromContext(ctx)
+
 	_, err := r.pool.Exec(ctx,
 		`DELETE FROM refresh_tokens WHERE token_hash = $1`,
 		tokenHash,
 	)
-	return err
+	if err != nil {
+		log.Errorw("delete refresh token failed", "error", err)
+		return err
+	}
+
+	log.Debugw("refresh token deleted")
+	return nil
 }
 
 func (r *PgUserRepository) DeleteUserRefreshTokens(ctx context.Context, userID string) error {
+	log := trace.FromContext(ctx)
+
 	_, err := r.pool.Exec(ctx,
 		`DELETE FROM refresh_tokens WHERE user_id = $1`,
 		userID,
 	)
-	return err
+	if err != nil {
+		log.Errorw("delete user refresh tokens failed", "user_id", userID, "error", err)
+		return err
+	}
+
+	log.Debugw("user refresh tokens deleted", "user_id", userID)
+	return nil
 }