Validate dev-login input: reject empty email and invalid role

Empty-string email passed model binding but created a ghost user with no
identity. Invalid role strings (e.g. "SuperHero") silently defaulted to
Admin, granting unintended elevated access.

Now returns 400 for both cases. Default role changed from Admin to
Dispatcher (least privilege).

api/src/ProposalSystem.Api/Controllers/AuthController.cs

@@ -99,7 +99,13 @@ public class AuthController : ControllerBase
         if (string.IsNullOrEmpty(signingKey))
             return StatusCode(500, new { message = "Dev signing key not configured" });
 
-        var role = Enum.TryParse<UserRole>(request.Role, true, out var parsed) ? parsed : UserRole.Admin;
+        if (string.IsNullOrWhiteSpace(request.Email))
+            return BadRequest(new { message = "Email is required" });
+
+        if (!string.IsNullOrEmpty(request.Role) && !Enum.TryParse<UserRole>(request.Role, true, out _))
+            return BadRequest(new { message = $"Invalid role: '{request.Role}'. Valid roles are: Dispatcher, Admin, SysAdmin" });
+
+        var role = Enum.TryParse<UserRole>(request.Role, true, out var parsed) ? parsed : UserRole.Dispatcher;
 
         var user = await _db.Users.FirstOrDefaultAsync(u => u.Email == request.Email, ct);
         if (user == null)