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/ILlmProvider.cs

@@ -0,0 +1,13 @@
+namespace TarninaHub.Application.Matchmaking;
+
+public interface ILlmProvider
+{
+    /// <summary>True when a real provider is configured (API key etc.).</summary>
+    bool IsConfigured { get; }
+
+    /// <summary>Returns the completion text or null when unavailable.</summary>
+    Task<string?> CompleteAsync(
+        string systemPrompt,
+        string userPrompt,
+        CancellationToken cancellationToken);
+}