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/TrendsController.cs

@@ -0,0 +1,37 @@
+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/trends")]
+[Authorize(Policy = Policies.OperatorOrAdmin)]
+public class TrendsController : ControllerBase
+{
+    private readonly ITrendService _trends;
+
+    public TrendsController(ITrendService trends)
+    {
+        _trends = trends;
+    }
+
+    [HttpGet]
+    public async Task<ActionResult<TrendsResponse>> Get(
+        [FromQuery] string? challengeId,
+        [FromQuery] DateTimeOffset? from,
+        [FromQuery] DateTimeOffset? to,
+        CancellationToken cancellationToken)
+    {
+        var rangeEnd = to ?? DateTimeOffset.UtcNow;
+        var rangeStart = from ?? rangeEnd.AddDays(-30);
+
+        var trends = await _trends.GetAsync(challengeId, rangeStart, rangeEnd, cancellationToken);
+
+        return Ok(trends);
+    }
+}