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/Application/Matchmaking/ILlmRequestLimiter.cs

@@ -0,0 +1,10 @@
+namespace TarninaHub.Application.Matchmaking;
+
+public interface ILlmRequestLimiter
+{
+    /// <summary>
+    /// Tries to reserve one LLM request within the current window.
+    /// Returns false when the limit is exceeded (caller should fall back).
+    /// </summary>
+    bool TryAcquire();
+}