feat(backend): add knowledge base, innovation library and trends modules

Add knowledge item and innovation CRUD with search/filtering, published-only visibility for non-staff, admin publish/update, challenge dictionary, problem aggregation with trends (admin only) and canonical embedding text preparation for matchmaking.

backend/Infrastructure/Persistence/ChallengeService.cs

@@ -0,0 +1,29 @@
+using SurrealDb.Net;
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Infrastructure.Persistence;
+
+public sealed class ChallengeService : IChallengeService
+{
+    private readonly ISurrealDbClient _client;
+
+    public ChallengeService(ISurrealDbClient client)
+    {
+        _client = client;
+    }
+
+    public async Task<IReadOnlyList<SocialChallengeResponse>> ListAsync(CancellationToken cancellationToken)
+    {
+        var response = await _client.RawQuery(
+            "SELECT type::string(id) AS id, name, description, order FROM social_challenge ORDER BY order;",
+            cancellationToken: cancellationToken);
+        response.EnsureAllOks();
+
+        var rows = response.GetValue<List<ChallengeRow>>(0) ?? [];
+
+        return rows
+            .Select(row => new SocialChallengeResponse(row.Id, row.Name, row.Description, row.Order))
+            .ToList();
+    }
+}