feat: Gemini als Provider hinzugefügt (Free-Tier-Modelle hardcodiert)

- OpenAiService: Gemini-Provider mit Base-URL generativelanguage.googleapis.com/v1beta/openai
- GeminiApiKey aus IConfiguration (appsettings.json / appsettings.Production.json)
- Gemini Free-Tier-Modelle hardcodiert: 2.5 Pro, 2.5 Flash, 2.5 Flash Lite
- ModelsEndpoint: /api/models/gemini Endpoint
- Frontend: geminiOptgroup dynamisch befüllt
- ProviderKeys als static HashSet (für ChatService.ResolveModelId)

Program.cs

@@ -70,7 +70,7 @@ builder.Services.AddSingleton<AnthropicService>(sp =>
 
 // OpenAI-kompatibler Service (Nvidia etc.)
 builder.Services.AddSingleton<OpenAiService>(sp =>
-    new OpenAiService(sp.GetRequiredService<IHttpClientFactory>(), dbService, sp.GetRequiredService<ToolService>()));
+    new OpenAiService(sp.GetRequiredService<IHttpClientFactory>(), dbService, sp.GetRequiredService<ToolService>(), builder.Configuration));
 
 // Telegram Bot (pro Session – Token aus Session-Config)
 var maxIterations = builder.Configuration.GetValue<int>("MaxIterations", 100);