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/Application/Abstractions/IChallengeService.cs

@@ -0,0 +1,8 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface IChallengeService
+{
+    Task<IReadOnlyList<SocialChallengeResponse>> ListAsync(CancellationToken cancellationToken);
+}