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/NullLlmProvider.cs

@@ -0,0 +1,15 @@
+namespace TarninaHub.Application.Matchmaking;
+
+/// <summary>
+/// Default provider used when no external LLM is configured. Keeps the system
+/// fully offline/deterministic; matchmaking falls back to template rationales.
+/// </summary>
+public sealed class NullLlmProvider : ILlmProvider
+{
+    public bool IsConfigured => false;
+
+    public Task<string?> CompleteAsync(
+        string systemPrompt,
+        string userPrompt,
+        CancellationToken cancellationToken) => Task.FromResult<string?>(null);
+}