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/Text/EmbeddingTextBuilder.cs

@@ -0,0 +1,35 @@
+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);
+    }
+}