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/IInnovationService.cs

@@ -0,0 +1,22 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface IInnovationService
+{
+    Task<IReadOnlyList<InnovationResponse>> ListAsync(
+        string? challengeId,
+        string? stage,
+        string? query,
+        bool onlyPublished,
+        CancellationToken cancellationToken);
+
+    Task<InnovationResponse?> GetAsync(string innovationId, CancellationToken cancellationToken);
+
+    Task<InnovationResponse> CreateAsync(InnovationRequest request, CancellationToken cancellationToken);
+
+    Task<InnovationResponse?> UpdateAsync(
+        string innovationId,
+        InnovationRequest request,
+        CancellationToken cancellationToken);
+}