Harden request validation and admin UX cues

Backend/Endpoints/PublicNominationEndpoints.cs

@@ -11,14 +11,21 @@ public static partial class PublicEndpoints
 {
     private static async Task<IResult> CreateNomination(
         HttpContext context,
-        CreateNominationRequest request,
+        CreateNominationRequest? request,
         AwardsDbContext db,
         IUserSessionService userSessionService,
         IRiskFlagService riskFlagService,
         IRiskRuleService riskRuleService,
         NominationEnrichmentService nominationEnrichmentService)
     {
-        var submittedNominations = NormalizeSubmittedNominations(request);
+        var validationError = ValidateCreateNominationRequest(request);
+        if (validationError is not null)
+        {
+            return validationError;
+        }
+
+        var validatedRequest = request!;
+        var submittedNominations = NormalizeSubmittedNominations(validatedRequest);
 
         if (submittedNominations.Length == 0)
         {
@@ -66,14 +73,14 @@ public static partial class PublicEndpoints
             return Results.BadRequest(new { message = "Dieser Link kann nicht nominiert werden. Bitte reiche einen direkten Kanal- oder Profil-Link ein." });
         }
 
-        var season = await db.Seasons.FirstOrDefaultAsync(item => item.Year == request.Year);
+        var season = await db.Seasons.FirstOrDefaultAsync(item => item.Year == validatedRequest.Year);
 
         if (season is null)
         {
             return Results.BadRequest(new { message = "The selected season does not exist." });
         }
 
-        var categoryGroupName = await ResolveCategoryGroupNameAsync(db, season.Id, request, context.RequestAborted);
+        var categoryGroupName = await ResolveCategoryGroupNameAsync(db, season.Id, validatedRequest, context.RequestAborted);
         if (string.IsNullOrWhiteSpace(categoryGroupName))
         {
             return Results.BadRequest(new { message = "The selected category group does not exist for this season." });
@@ -102,7 +109,7 @@ public static partial class PublicEndpoints
             return nominationSeasonResolution.Result;
         }
 
-        var submitterIdResult = await ResolveSubmitterIdAsync(context, request.TwitchUserId, userSessionService);
+        var submitterIdResult = await ResolveSubmitterIdAsync(context, validatedRequest.TwitchUserId, userSessionService);
         if (submitterIdResult.Result is not null)
         {
             return submitterIdResult.Result;