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/RationaleBuilder.cs

@@ -0,0 +1,29 @@
+namespace TarninaHub.Application.Matchmaking;
+
+/// <summary>
+/// Deterministic, offline explanation of "why this fits". Used as the default
+/// and as a fallback when the LLM is unavailable or rate-limited.
+/// </summary>
+public static class RationaleBuilder
+{
+    public static string Build(bool challengeMatch, bool locationMatch, double score)
+    {
+        var reasons = new List<string>();
+
+        if (challengeMatch)
+        {
+            reasons.Add("ten sam obszar problemowy");
+        }
+
+        if (locationMatch)
+        {
+            reasons.Add("zbliżona lokalizacja");
+        }
+
+        reasons.Add(score >= 0.3
+            ? "wysoka zbieżność opisu potrzeby"
+            : "zbieżność semantyczna opisu");
+
+        return "Proponowane dopasowanie: " + string.Join(", ", reasons) + ".";
+    }
+}