* Extended functionalities

* Added different UIs

MealPlan.Api/Services/AI/IOpenAiChatService.cs

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+namespace MealPlan.Api.Services.AI;
+
+public interface IOpenAiChatService
+{
+    bool IsConfigured { get; }
+
+    Task<string> CompleteJsonAsync(
+        string systemPrompt,
+        string userPrompt,
+        OpenAiUseCase useCase = OpenAiUseCase.Default,
+        CancellationToken ct = default);
+}

MealPlan.Api/Services/AI/OpenAiChatService.cs

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+using System.Net.Http.Headers;
+using System.Text;
+using System.Text.Json;
+using Microsoft.Extensions.Options;
+
+namespace MealPlan.Api.Services.AI;
+
+public class OpenAiChatService : IOpenAiChatService
+{
+    private readonly HttpClient _http;
+    private readonly OpenAiOptions _options;
+    private readonly ILogger<OpenAiChatService> _logger;
+
+    public OpenAiChatService(HttpClient http, IOptions<OpenAiOptions> options, ILogger<OpenAiChatService> logger)
+    {
+        _http = http;
+        _options = options.Value;
+        _logger = logger;
+    }
+
+    public bool IsConfigured => _options.IsConfigured;
+
+    public async Task<string> CompleteJsonAsync(
+        string systemPrompt,
+        string userPrompt,
+        OpenAiUseCase useCase = OpenAiUseCase.Default,
+        CancellationToken ct = default)
+    {
+        if (!IsConfigured)
+        {
+            throw new InvalidOperationException(
+                "OpenAI is not configured. Set OpenAI__ApiKey in appsettings.Local.json.");
+        }
+
+        var model = _options.ResolveModel(useCase);
+        var payload = new
+        {
+            model,
+            max_tokens = _options.MaxTokens,
+            response_format = new { type = "json_object" },
+            messages = new[]
+            {
+                new { role = "system", content = systemPrompt },
+                new { role = "user", content = userPrompt },
+            },
+        };
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, "https://api.openai.com/v1/chat/completions");
+        request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", _options.ApiKey);
+        request.Content = new StringContent(JsonSerializer.Serialize(payload), Encoding.UTF8, "application/json");
+
+        using var response = await _http.SendAsync(request, ct);
+        var body = await response.Content.ReadAsStringAsync(ct);
+        if (!response.IsSuccessStatusCode)
+        {
+            _logger.LogWarning("OpenAI API error {Status}: {Body}", response.StatusCode, body);
+            throw new InvalidOperationException("OpenAI request failed. Check API key and quota.");
+        }
+
+        using var doc = JsonDocument.Parse(body);
+        var content = doc.RootElement
+            .GetProperty("choices")[0]
+            .GetProperty("message")
+            .GetProperty("content")
+            .GetString() ?? "{}";
+
+        return StripMarkdownFence(content);
+    }
+
+    private static string StripMarkdownFence(string content)
+    {
+        var trimmed = content.Trim();
+        if (!trimmed.StartsWith("```", StringComparison.Ordinal)) return trimmed;
+        var firstNewline = trimmed.IndexOf('\n');
+        if (firstNewline < 0) return trimmed;
+        var endFence = trimmed.LastIndexOf("```", StringComparison.Ordinal);
+        if (endFence <= firstNewline) return trimmed;
+        return trimmed[(firstNewline + 1)..endFence].Trim();
+    }
+}

MealPlan.Api/Services/AI/OpenAiOptions.cs

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+namespace MealPlan.Api.Services.AI;
+
+public enum OpenAiUseCase
+{
+    Default,
+    Macros,
+    Plan,
+    Recipe,
+}
+
+public class OpenAiOptions
+{
+    public const string SectionName = "OpenAI";
+
+    public string ApiKey { get; set; } = string.Empty;
+
+    /// <summary>Fallback when a task-specific model is not set.</summary>
+    public string Model { get; set; } = "gpt-4.1-mini";
+
+    /// <summary>Macro target refinement (diet generator step 2).</summary>
+    public string? MacrosModel { get; set; }
+
+    /// <summary>Weekly meal plan recipe selection (diet generator step 3).</summary>
+    public string? PlanModel { get; set; }
+
+    /// <summary>AI recipe draft generation and partial regen.</summary>
+    public string? RecipeModel { get; set; }
+
+    public int MaxTokens { get; set; } = 4096;
+
+    public bool IsConfigured => !string.IsNullOrWhiteSpace(ApiKey);
+
+    public string ResolveModel(OpenAiUseCase useCase) =>
+        useCase switch
+        {
+            OpenAiUseCase.Macros => FirstNonEmpty(MacrosModel, Model),
+            OpenAiUseCase.Plan => FirstNonEmpty(PlanModel, Model),
+            OpenAiUseCase.Recipe => FirstNonEmpty(RecipeModel, Model),
+            _ => Model,
+        };
+
+    private static string FirstNonEmpty(string? preferred, string fallback) =>
+        string.IsNullOrWhiteSpace(preferred) ? fallback : preferred;
+}