feat: migrate to Better Auth for authentication, update environment variables, and enhance database schema with accounts and sessions

Backend/types/express.d.ts

@@ -1,9 +1,17 @@
-import type { JwtPayload } from '../utils/jwt';
-
 declare global {
   namespace Express {
     interface Request {
-      user?: JwtPayload;
+      auth?: {
+        user: {
+          id: string;
+          email: string;
+          name?: string | null;
+        };
+        session: {
+          id: string;
+          userId: string;
+        };
+      };
     }
   }
 }