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);
+}

backend/Application/Abstractions/ISubmissionService.cs

@@ -0,0 +1,28 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface ISubmissionService
+{
+    Task<SubmissionResponse> CreateAsync(
+        string authorId,
+        CreateSubmissionRequest request,
+        CancellationToken cancellationToken);
+
+    Task<PagedResponse<SubmissionResponse>> ListAsync(
+        string viewerId,
+        bool isStaff,
+        string? status,
+        int page,
+        int pageSize,
+        CancellationToken cancellationToken);
+
+    Task<SubmissionResponse?> GetAsync(
+        string submissionId,
+        CancellationToken cancellationToken);
+
+    Task<SubmissionResponse?> UpdateStatusAsync(
+        string submissionId,
+        string newStatus,
+        CancellationToken cancellationToken);
+}

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);

backend/Application/Mappings/SubmissionStatusWire.cs

@@ -0,0 +1,36 @@
+using TarninaHub.Domain.Enums;
+
+namespace TarninaHub.Application.Mappings;
+
+public static class SubmissionStatusWire
+{
+    public static string ToWire(this SubmissionStatus status) => status switch
+    {
+        SubmissionStatus.New => "new",
+        SubmissionStatus.Triaged => "triaged",
+        SubmissionStatus.Matched => "matched",
+        SubmissionStatus.InProgress => "in_progress",
+        SubmissionStatus.Closed => "closed",
+        SubmissionStatus.Rejected => "rejected",
+        _ => throw new ArgumentOutOfRangeException(nameof(status), status, null),
+    };
+
+    public static SubmissionStatus? Parse(string? value)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+        {
+            return null;
+        }
+
+        return value switch
+        {
+            "new" => SubmissionStatus.New,
+            "triaged" => SubmissionStatus.Triaged,
+            "matched" => SubmissionStatus.Matched,
+            "in_progress" => SubmissionStatus.InProgress,
+            "closed" => SubmissionStatus.Closed,
+            "rejected" => SubmissionStatus.Rejected,
+            _ => null,
+        };
+    }
+}

backend/Application/Text/KeywordExtractor.cs

@@ -0,0 +1,70 @@
+using System.Text;
+
+namespace TarninaHub.Application.Text;
+
+public static class KeywordExtractor
+{
+    private static readonly HashSet<string> StopWords = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "i", "w", "we", "na", "z", "do", "nie", "że", "się", "jest", "to", "dla", "od", "po",
+        "o", "a", "ale", "jak", "co", "oraz", "przez", "przy", "tak", "już", "czy", "ich",
+        "the", "and", "for", "with", "this", "that", "are", "was",
+    };
+
+    public static IReadOnlyList<string> Extract(string text, int max = 8)
+    {
+        if (string.IsNullOrWhiteSpace(text) || max <= 0)
+        {
+            return [];
+        }
+
+        var counts = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);
+        var order = new List<string>();
+
+        foreach (var rawToken in Tokenize(text))
+        {
+            var token = rawToken.ToLowerInvariant();
+
+            if (token.Length < 3 || StopWords.Contains(token))
+            {
+                continue;
+            }
+
+            if (!counts.TryGetValue(token, out var count))
+            {
+                order.Add(token);
+            }
+
+            counts[token] = count + 1;
+        }
+
+        return order
+            .OrderByDescending(token => counts[token])
+            .ThenBy(token => order.IndexOf(token))
+            .Take(max)
+            .ToList();
+    }
+
+    private static IEnumerable<string> Tokenize(string text)
+    {
+        var buffer = new StringBuilder();
+
+        foreach (var character in text)
+        {
+            if (char.IsLetter(character))
+            {
+                buffer.Append(character);
+            }
+            else if (buffer.Length > 0)
+            {
+                yield return buffer.ToString();
+                buffer.Clear();
+            }
+        }
+
+        if (buffer.Length > 0)
+        {
+            yield return buffer.ToString();
+        }
+    }
+}