feat: add role-based access control middleware and auth routes

This commit implements the role-based access control as per the project's security requirements. It includes:
- A new middleware 'requireRole' that checks user roles for protected endpoints
- Updated auth routes with role protection
- Auth controller with proper registration and login logic including JWT token generation
- Default user role assignment during registration

backend/src/middleware/requireRole.js

@@ -0,0 +1,20 @@
+/**
+ * Middleware to require a specific role for an endpoint.
+ * @param {string[]} allowedRoles - Array of roles allowed to access the endpoint.
+ * @returns {function} Express middleware function.
+ */
+export default function requireRole(allowedRoles) {
+  return (req, res, next) => {
+    const userRole = req.user?.role;
+    
+    if (!userRole) {
+      return res.status(401).json({ error: 'Authorization required' });
+    }
+    
+    if (!allowedRoles.includes(userRole)) {
+      return res.status(403).json({ error: 'Insufficient permissions' });
+    }
+    
+    next();
+  };
+}