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);
+}

backend/Application/Abstractions/IInnovationService.cs

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+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);
+}

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);

backend/Application/Mappings/InnovationStageWire.cs

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+using TarninaHub.Domain.Enums;
+
+namespace TarninaHub.Application.Mappings;
+
+public static class InnovationStageWire
+{
+    private static readonly Dictionary<string, InnovationStage> ByWire = new(StringComparer.Ordinal)
+    {
+        ["idea"] = InnovationStage.Idea,
+        ["pilot"] = InnovationStage.Pilot,
+        ["scaling"] = InnovationStage.Scaling,
+        ["service"] = InnovationStage.Service,
+    };
+
+    public static bool IsValid(string? value) =>
+        !string.IsNullOrWhiteSpace(value) && ByWire.ContainsKey(value);
+
+    public static string ToWire(this InnovationStage stage) => stage switch
+    {
+        InnovationStage.Idea => "idea",
+        InnovationStage.Pilot => "pilot",
+        InnovationStage.Scaling => "scaling",
+        InnovationStage.Service => "service",
+        _ => throw new ArgumentOutOfRangeException(nameof(stage), stage, null),
+    };
+
+    public static InnovationStage? Parse(string? value) =>
+        value is not null && ByWire.TryGetValue(value, out var stage) ? stage : null;
+}

backend/Application/Text/EmbeddingTextBuilder.cs

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+namespace TarninaHub.Application.Text;
+
+/// <summary>
+/// Builds the canonical text used to compute an innovation embedding
+/// (matchmaking, steps 14-15).
+/// </summary>
+public static class EmbeddingTextBuilder
+{
+    public static string Build(string title, string? summary, string? description, string? challengeName = null)
+    {
+        var parts = new List<string>();
+
+        if (!string.IsNullOrWhiteSpace(title))
+        {
+            parts.Add(title.Trim());
+        }
+
+        if (!string.IsNullOrWhiteSpace(summary))
+        {
+            parts.Add(summary.Trim());
+        }
+
+        if (!string.IsNullOrWhiteSpace(description))
+        {
+            parts.Add(description.Trim());
+        }
+
+        if (!string.IsNullOrWhiteSpace(challengeName))
+        {
+            parts.Add(challengeName.Trim());
+        }
+
+        return string.Join("\n", parts);
+    }
+}