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/MatchScoring.cs

@@ -0,0 +1,37 @@
+namespace TarninaHub.Application.Matchmaking;
+
+/// <summary>
+/// Combines semantic similarity with context signals (area, location) and
+/// availability into a final 0..1 match score.
+/// </summary>
+public static class MatchScoring
+{
+    private const double ChallengeBoost = 0.10;
+    private const double LocationBoost = 0.05;
+
+    public static double Combine(
+        double semanticScore,
+        bool challengeMatch,
+        bool locationMatch,
+        bool available)
+    {
+        if (!available)
+        {
+            return 0;
+        }
+
+        var score = semanticScore;
+
+        if (challengeMatch)
+        {
+            score += ChallengeBoost;
+        }
+
+        if (locationMatch)
+        {
+            score += LocationBoost;
+        }
+
+        return Math.Clamp(score, 0, 1);
+    }
+}