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/dto/sessionInfo.go

@@ -0,0 +1,27 @@
+
+// Copyright (c) 2025 Nikolai Papin
+//
+// This file is part of Easywish
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
+// the GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package dto
+
+import "easywish/internal/utils/enums"
+
+type SessionInfo struct {
+	Username string
+	Session string
+	Role enums.Role 
+}