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

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

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);
+}

backend/Application/Abstractions/IKnowledgeItemService.cs

@@ -0,0 +1,24 @@
+using TarninaHub.Application.Dtos;
+
+namespace TarninaHub.Application.Abstractions;
+
+public interface IKnowledgeItemService
+{
+    Task<IReadOnlyList<KnowledgeItemResponse>> ListAsync(
+        string? kind,
+        string? challengeId,
+        string? query,
+        bool onlyPublished,
+        CancellationToken cancellationToken);
+
+    Task<KnowledgeItemResponse?> GetAsync(string knowledgeItemId, CancellationToken cancellationToken);
+
+    Task<KnowledgeItemResponse> CreateAsync(KnowledgeItemRequest request, CancellationToken cancellationToken);
+
+    Task<KnowledgeItemResponse?> UpdateAsync(
+        string knowledgeItemId,
+        KnowledgeItemRequest request,
+        CancellationToken cancellationToken);
+
+    Task<KnowledgeItemResponse?> PublishAsync(string knowledgeItemId, CancellationToken cancellationToken);
+}

backend/Application/Abstractions/ITrendService.cs

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