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/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();
+        }
+    }
+}