refactor: move Claims type from auth to new identity package

Extract Claims struct from auth/types.go into dedicated identity package for better separation of concerns. Update all imports and usages across auth service, token handling, and request context utilities.

backend/internal/identity/claims.go

@@ -0,0 +1,10 @@
+package identity
+
+import "github.com/google/uuid"
+
+type Claims struct {
+	UserID   uuid.UUID `json:"user_id"`
+	Username string    `json:"username"`
+	Role     string    `json:"role"`
+	Session  uuid.UUID `json:"session_id"`
+}