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/Abstractions/ISubmissionEventPublisher.cs

@@ -0,0 +1,13 @@
+namespace TarninaHub.Application.Abstractions;
+
+public sealed record SubmissionEvent(
+    string Type,
+    string SubmissionId,
+    string? ActorId,
+    string Status,
+    DateTimeOffset At);
+
+public interface ISubmissionEventPublisher
+{
+    Task PublishAsync(SubmissionEvent submissionEvent, CancellationToken cancellationToken);
+}