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)

Endpoints/ModelsEndpoint.cs

@@ -12,6 +12,7 @@ public static class ModelsEndpoint
     {
         app.MapGet("/api/models/nvidia",       HandleNvidiaModels);
         app.MapGet("/api/models/pollinations", HandlePollinationsModels);
+        app.MapGet("/api/models/gemini",       HandleGeminiModels);
     }
 
     private static IResult HandleNvidiaModels(OpenAiService openAiService)
@@ -29,4 +30,12 @@ public static class ModelsEndpoint
             .ToList();
         return Results.Json(new { models });
     }
+
+    private static IResult HandleGeminiModels(OpenAiService openAiService)
+    {
+        var models = openAiService.GetGeminiModels()
+            .Select(m => new { id = m.Id, displayName = m.DisplayName })
+            .ToList();
+        return Results.Json(new { models });
+    }
 }