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/ITrendService.cs

@@ -0,0 +1,12 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface ITrendService
+{
+    Task<TrendsResponse> GetAsync(
+        string? challengeId,
+        DateTimeOffset from,
+        DateTimeOffset to,
+        CancellationToken cancellationToken);
+}