refactor(api): enforce protocol-neutral service boundaries

- refactor: move HTTP status and DTO concerns out of model and service
  layers into API and handler code.
- feat: add admin service boundary and route auth through services
  instead of direct repository access.
- test: add architecture and error tests covering package boundaries,
  redaction, and service behavior.
- docs: record the protocol-neutral boundary decision and update
  architecture and roadmap notes.

internal/handler/auth.go

@@ -3,10 +3,12 @@ package handler
 import (
 	"log/slog"
 	"net/http"
+	"time"
 
 	"github.com/gin-gonic/gin"
 
 	"github.com/dhao2001/mygo/internal/api"
+	"github.com/dhao2001/mygo/internal/model"
 	"github.com/dhao2001/mygo/internal/service"
 )
 
@@ -35,6 +37,20 @@ type tokenRequest struct {
 	RefreshToken string `json:"refresh_token" binding:"required"`
 }
 
+type userResponse struct {
+	ID        string    `json:"id"`
+	Username  string    `json:"username"`
+	Email     string    `json:"email"`
+	IsAdmin   bool      `json:"is_admin"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+type tokenPairResponse struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token"`
+}
+
 // Register handles POST /api/v1/auth/register.
 func (h *AuthHandler) Register(c *gin.Context) {
 	var req registerRequest
@@ -50,7 +66,7 @@ func (h *AuthHandler) Register(c *gin.Context) {
 		return
 	}
 
-	c.JSON(http.StatusCreated, user)
+	c.JSON(http.StatusCreated, toUserResponse(user))
 }
 
 // Login handles POST /api/v1/auth/login.
@@ -68,7 +84,7 @@ func (h *AuthHandler) Login(c *gin.Context) {
 		return
 	}
 
-	c.JSON(http.StatusOK, pair)
+	c.JSON(http.StatusOK, toTokenPairResponse(pair))
 }
 
 // Refresh handles POST /api/v1/auth/refresh.
@@ -86,7 +102,7 @@ func (h *AuthHandler) Refresh(c *gin.Context) {
 		return
 	}
 
-	c.JSON(http.StatusOK, pair)
+	c.JSON(http.StatusOK, toTokenPairResponse(pair))
 }
 
 // Logout handles POST /api/v1/auth/logout.
@@ -105,3 +121,21 @@ func (h *AuthHandler) Logout(c *gin.Context) {
 
 	c.Status(http.StatusOK)
 }
+
+func toUserResponse(user *model.User) userResponse {
+	return userResponse{
+		ID:        user.ID,
+		Username:  user.Username,
+		Email:     user.Email,
+		IsAdmin:   user.IsAdmin,
+		CreatedAt: user.CreatedAt,
+		UpdatedAt: user.UpdatedAt,
+	}
+}
+
+func toTokenPairResponse(pair *service.TokenPair) tokenPairResponse {
+	return tokenPairResponse{
+		AccessToken:  pair.AccessToken,
+		RefreshToken: pair.RefreshToken,
+	}
+}