refactor: introduce DTOs for claims, session, and request handling

feat: add token validation service method
refactor: update middleware to use structured DTOs
feat: implement session info propagation through context
refactor: replace ad-hoc structs with DTOs in middleware
chore: organize auth-related data structures

backend/internal/middleware/auth.go

@@ -21,6 +21,7 @@ import (
 	"context"
 	"easywish/config"
 	"easywish/internal/database"
+	"easywish/internal/dto"
 	"easywish/internal/utils/enums"
 	"errors"
 	"fmt"
@@ -34,14 +35,6 @@ import (
 	"go.uber.org/zap"
 )
 
-type Claims struct {
-	Username string     				`json:"username"`
-	Role     enums.Role 			  `json:"role"`
-	Type 		 enums.JwtTokenType `json:"type"`
-	Session  string 					  `json:"session"`	
-	jwt.RegisteredClaims
-}
-
 // XXX: cluttered; move cache & database check to auth service
 func AuthMiddleware(log *zap.Logger, dbctx database.DbContext, redisClient *redis.Client) gin.HandlerFunc {
 	return func(c *gin.Context) {
@@ -49,8 +42,12 @@ func AuthMiddleware(log *zap.Logger, dbctx database.DbContext, redisClient *redi
 		authHeader := c.GetHeader("Authorization")
 
 		if authHeader == "" {
-			c.Set("username", nil)
-			c.Set("role", enums.GuestRole)
+
+			c.Set("session_info", dto.SessionInfo{
+				Username: "",
+				Session: "",
+				Role: enums.GuestRole},
+			)
 			c.Next()
 			return
 		}
@@ -59,7 +56,7 @@ func AuthMiddleware(log *zap.Logger, dbctx database.DbContext, redisClient *redi
 
 		token, err := jwt.ParseWithClaims(
 			tokenString, 
-			&Claims{}, 
+			&dto.UserClaims{}, 
 			func(token *jwt.Token) (any, error) {
 				if _, ok := token.Method.(*jwt.SigningMethodHMAC); !ok {
 					return nil, fmt.Errorf("unexpected signing method: %v", token.Header["alg"])
@@ -77,7 +74,7 @@ func AuthMiddleware(log *zap.Logger, dbctx database.DbContext, redisClient *redi
 			return
 		}
 
-		if claims, ok := token.Claims.(*Claims); ok && token.Valid {
+		if claims, ok := token.Claims.(*dto.UserClaims); ok && token.Valid {
 
 			if claims.Type != enums.JwtAccessTokenType {
 				c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "Not an access token"})
@@ -142,8 +139,12 @@ func AuthMiddleware(log *zap.Logger, dbctx database.DbContext, redisClient *redi
 				return
 			}
 
-			c.Set("username", claims.Username)
-			c.Set("role", claims.Role)
+			c.Set("session_info", dto.SessionInfo{
+				Username: claims.Username,
+				Session: claims.Session,
+				Role: claims.Role,
+			})
+
 			c.Next()
 		} else {
 			c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "Invalid claims"})