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/Controllers/SocialChallengesController.cs

@@ -0,0 +1,31 @@
+using Asp.Versioning;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+using TarninaHub.Application.Abstractions;
+using TarninaHub.Application.Dtos;
+using TarninaHub.Application.Security;
+
+namespace TarninaHub.Controllers;
+
+[ApiController]
+[ApiVersion("1.0")]
+[Route("api/social-challenges")]
+[Authorize]
+public class SocialChallengesController : ControllerBase
+{
+    private readonly IChallengeService _challenges;
+
+    public SocialChallengesController(IChallengeService challenges)
+    {
+        _challenges = challenges;
+    }
+
+    [HttpGet]
+    public async Task<ActionResult<IReadOnlyList<SocialChallengeResponse>>> List(
+        CancellationToken cancellationToken)
+    {
+        var challenges = await _challenges.ListAsync(cancellationToken);
+
+        return Ok(challenges);
+    }
+}