feat(backend): add LLM provider abstraction, contextual ranking and explanations

Add a pluggable LLM provider with an offline template fallback, prompt builder, contextual match scoring (area/location boosts, availability), per-solution explanations and in-memory request limiting for cost control.

backend/Infrastructure/Llm/InMemoryLlmRequestLimiter.cs

@@ -0,0 +1,42 @@
+using TarninaHub.Application.Matchmaking;
+
+namespace TarninaHub.Infrastructure.Llm;
+
+/// <summary>
+/// In-memory sliding-window request limiter (per instance) for LLM cost control.
+/// </summary>
+public sealed class InMemoryLlmRequestLimiter : ILlmRequestLimiter
+{
+    private readonly int _maxPerWindow;
+    private readonly TimeSpan _window;
+    private readonly Queue<DateTimeOffset> _timestamps = new();
+    private readonly Lock _gate = new();
+
+    public InMemoryLlmRequestLimiter(int maxRequestsPerMinute, TimeSpan? window = null)
+    {
+        _maxPerWindow = Math.Max(1, maxRequestsPerMinute);
+        _window = window ?? TimeSpan.FromMinutes(1);
+    }
+
+    public bool TryAcquire()
+    {
+        var now = DateTimeOffset.UtcNow;
+
+        lock (_gate)
+        {
+            while (_timestamps.Count > 0 && now - _timestamps.Peek() > _window)
+            {
+                _timestamps.Dequeue();
+            }
+
+            if (_timestamps.Count >= _maxPerWindow)
+            {
+                return false;
+            }
+
+            _timestamps.Enqueue(now);
+
+            return true;
+        }
+    }
+}