feat(backend): add problem/need submission module with workflow and events

Add submission CRUD with pagination and role scoping, challenge/location binding, automatic keyword extraction, a validated status workflow, and domain events for matchmaking and notifications. Create a problem record together with each submission.

backend/Application/Dtos/PagedResponse.cs

@@ -0,0 +1,7 @@
+namespace TarninaHub.Application.Dtos;
+
+public sealed record PagedResponse<T>(
+    IReadOnlyList<T> Items,
+    int Page,
+    int PageSize,
+    int Total);