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/Infrastructure/Events/LoggingSubmissionEventPublisher.cs

@@ -0,0 +1,30 @@
+using Microsoft.Extensions.Logging;
+using TarninaHub.Application.Abstractions;
+
+namespace TarninaHub.Infrastructure.Events;
+
+/// <summary>
+/// Placeholder publisher: logs domain events. Will be replaced/wired to
+/// matchmaking (steps 14-15) and notifications (step 17).
+/// </summary>
+public sealed class LoggingSubmissionEventPublisher : ISubmissionEventPublisher
+{
+    private readonly ILogger<LoggingSubmissionEventPublisher> _logger;
+
+    public LoggingSubmissionEventPublisher(ILogger<LoggingSubmissionEventPublisher> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task PublishAsync(SubmissionEvent submissionEvent, CancellationToken cancellationToken)
+    {
+        _logger.LogInformation(
+            "Domain event {EventType} for {SubmissionId} (status: {Status}, actor: {ActorId})",
+            submissionEvent.Type,
+            submissionEvent.SubmissionId,
+            submissionEvent.Status,
+            submissionEvent.ActorId ?? "system");
+
+        return Task.CompletedTask;
+    }
+}