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/Dtos/InnovationDtos.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace TarninaHub.Application.Dtos;
+
+public sealed record InnovationRequest(
+    [Required, StringLength(200, MinimumLength = 3)] string Title,
+    [StringLength(2000)] string? Summary,
+    [StringLength(8000)] string? Description,
+    [Required] string ChallengeId,
+    [Required] string Stage,
+    string? Cost,
+    IReadOnlyDictionary<string, object?>? Contact,
+    IReadOnlyList<IReadOnlyDictionary<string, object?>>? Media,
+    bool Published);
+
+public sealed record InnovationResponse(
+    string Id,
+    string Title,
+    string Summary,
+    string Description,
+    string ChallengeId,
+    string Stage,
+    string? Cost,
+    IReadOnlyDictionary<string, object?>? Contact,
+    IReadOnlyList<IReadOnlyDictionary<string, object?>> Media,
+    bool Published,
+    DateTimeOffset? UpdatedAt);

backend/Application/Dtos/KnowledgeDtos.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace TarninaHub.Application.Dtos;
+
+public sealed record KnowledgeItemRequest(
+    [Required, StringLength(200, MinimumLength = 3)] string Title,
+    [Required] string Kind,
+    string? ChallengeId,
+    string? SourceUrl,
+    IReadOnlyList<IReadOnlyDictionary<string, object?>>? Attachments,
+    bool Published);
+
+public sealed record KnowledgeItemResponse(
+    string Id,
+    string Title,
+    string Kind,
+    string? ChallengeId,
+    string? SourceUrl,
+    IReadOnlyList<IReadOnlyDictionary<string, object?>> Attachments,
+    bool Published,
+    DateTimeOffset? UpdatedAt);
+
+public sealed record SocialChallengeResponse(
+    string Id,
+    string Name,
+    string Description,
+    int Order);

backend/Application/Dtos/TrendsDtos.cs

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+namespace TarninaHub.Application.Dtos;
+
+public sealed record TrendPoint(
+    string ChallengeId,
+    string ChallengeName,
+    int SubmissionCount);
+
+public sealed record TrendsResponse(
+    DateTimeOffset From,
+    DateTimeOffset To,
+    IReadOnlyList<TrendPoint> Points);