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/Infrastructure/Llm/LlmSettings.cs

@@ -0,0 +1,23 @@
+namespace TarninaHub.Infrastructure.Llm;
+
+public sealed class LlmSettings
+{
+    public bool Enabled { get; set; }
+
+    /// <summary>Provider identifier, e.g. "openai" (OpenAI-compatible chat API).</summary>
+    public string Provider { get; set; } = "openai";
+
+    public string? ApiKey { get; set; }
+
+    public string BaseUrl { get; set; } = "https://api.openai.com/v1";
+
+    public string Model { get; set; } = "gpt-4o-mini";
+
+    public int MaxTokens { get; set; } = 256;
+
+    public int MaxRequestsPerMinute { get; set; } = 30;
+
+    public int TimeoutSeconds { get; set; } = 20;
+
+    public bool IsConfigured => Enabled && !string.IsNullOrWhiteSpace(ApiKey);
+}