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);

backend/Application/Dtos/SubmissionDtos.cs

@@ -0,0 +1,26 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace TarninaHub.Application.Dtos;
+
+public sealed record CreateSubmissionRequest(
+    [Required, StringLength(200, MinimumLength = 3)] string Title,
+    [Required, StringLength(4000, MinimumLength = 10)] string Description,
+    [Required] string ChallengeId,
+    [Required] string LocationId,
+    IReadOnlyList<string>? Keywords);
+
+public sealed record UpdateSubmissionStatusRequest(
+    [Required] string Status);
+
+public sealed record SubmissionResponse(
+    string Id,
+    string ProblemId,
+    string Title,
+    string Description,
+    string ChallengeId,
+    string LocationId,
+    string AuthorId,
+    string Status,
+    IReadOnlyList<string> Keywords,
+    DateTimeOffset CreatedAt,
+    DateTimeOffset? UpdatedAt);