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/DependencyInjection.cs

@@ -5,6 +5,7 @@ using TarninaHub.Application.Abstractions;
 using TarninaHub.Application.Matchmaking;
 using TarninaHub.Infrastructure.Auth;
 using TarninaHub.Infrastructure.Events;
+using TarninaHub.Infrastructure.Llm;
 using TarninaHub.Infrastructure.Persistence;
 using TarninaHub.Infrastructure.Settings;
 
@@ -19,6 +20,23 @@ public static class DependencyInjection
         services.Configure<SurrealDbSettings>(configuration.GetSection("SurrealDb"));
         services.Configure<AuthSettings>(configuration.GetSection("Auth"));
 
+        var llmSettings = configuration.GetSection("Llm").Get<LlmSettings>() ?? new LlmSettings();
+        services.Configure<LlmSettings>(configuration.GetSection("Llm"));
+        services.AddSingleton<ILlmRequestLimiter>(
+            new InMemoryLlmRequestLimiter(llmSettings.MaxRequestsPerMinute));
+
+        if (llmSettings.IsConfigured)
+        {
+            services.AddHttpClient<OpenAiCompatibleLlmProvider>(
+                client => client.Timeout = TimeSpan.FromSeconds(llmSettings.TimeoutSeconds));
+            services.AddSingleton<ILlmProvider>(
+                provider => provider.GetRequiredService<OpenAiCompatibleLlmProvider>());
+        }
+        else
+        {
+            services.AddSingleton<ILlmProvider, NullLlmProvider>();
+        }
+
         var settings = configuration.GetSection("SurrealDb").Get<SurrealDbSettings>()
             ?? new SurrealDbSettings();