feat(api): .NET 10 multi-tenant REST API

Full backend implementation:
- Multi-tenant cafe/restaurant management (menus, orders, tables, staff)
- POS order flow with ZarinPal and Snappfood payment integration
- OTP authentication via Kavenegar SMS
- QR digital menu with public discover/finder endpoints
- Customer loyalty, coupons, CRM
- PostgreSQL via EF Core, Redis for caching/sessions
- Background jobs, webhook handlers
- Full migration history

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/Meezi.API/Services/OpenAiChatService.cs

@@ -0,0 +1,117 @@
+using System.Net.Http.Headers;
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Meezi.Core.Interfaces;
+using Meezi.Core.Platform;
+
+namespace Meezi.API.Services;
+
+public interface IOpenAiChatService
+{
+    Task<bool> IsConfiguredForCoffeeAdvisorAsync(CancellationToken cancellationToken = default);
+    Task<string?> CompleteJsonAsync(string systemPrompt, string userPrompt, CancellationToken cancellationToken = default);
+}
+
+public class OpenAiChatService : IOpenAiChatService
+{
+    private static readonly JsonSerializerOptions JsonOpts = new()
+    {
+        PropertyNameCaseInsensitive = true,
+        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+    };
+
+    private readonly IHttpClientFactory _httpClientFactory;
+    private readonly IPlatformRuntimeConfig _platform;
+    private readonly IConfiguration _configuration;
+    private readonly ILogger<OpenAiChatService> _logger;
+
+    public OpenAiChatService(
+        IHttpClientFactory httpClientFactory,
+        IPlatformRuntimeConfig platform,
+        IConfiguration configuration,
+        ILogger<OpenAiChatService> logger)
+    {
+        _httpClientFactory = httpClientFactory;
+        _platform = platform;
+        _configuration = configuration;
+        _logger = logger;
+    }
+
+    public async Task<bool> IsConfiguredForCoffeeAdvisorAsync(CancellationToken cancellationToken = default)
+    {
+        if (!await IsCoffeeAdvisorEnabledAsync(cancellationToken))
+            return false;
+        return !string.IsNullOrWhiteSpace(await GetApiKeyAsync(cancellationToken));
+    }
+
+    public async Task<string?> CompleteJsonAsync(
+        string systemPrompt,
+        string userPrompt,
+        CancellationToken cancellationToken = default)
+    {
+        var apiKey = await GetApiKeyAsync(cancellationToken);
+        if (string.IsNullOrWhiteSpace(apiKey))
+            return null;
+
+        var model = await GetModelAsync(cancellationToken);
+        var body = JsonSerializer.Serialize(new
+        {
+            model,
+            temperature = 0.4,
+            response_format = new { type = "json_object" },
+            messages = new object[]
+            {
+                new { role = "system", content = systemPrompt },
+                new { role = "user", content = userPrompt }
+            }
+        }, JsonOpts);
+
+        var client = _httpClientFactory.CreateClient("OpenAi");
+        using var req = new HttpRequestMessage(HttpMethod.Post, "https://api.openai.com/v1/chat/completions")
+        {
+            Content = new StringContent(body, Encoding.UTF8, "application/json")
+        };
+        req.Headers.Authorization = new AuthenticationHeaderValue("Bearer", apiKey);
+
+        using var res = await client.SendAsync(req, cancellationToken);
+        if (!res.IsSuccessStatusCode)
+        {
+            _logger.LogWarning("OpenAI chat failed with status {Status}", res.StatusCode);
+            return null;
+        }
+
+        await using var stream = await res.Content.ReadAsStreamAsync(cancellationToken);
+        var doc = await JsonDocument.ParseAsync(stream, cancellationToken: cancellationToken);
+        return doc.RootElement
+            .GetProperty("choices")[0]
+            .GetProperty("message")
+            .GetProperty("content")
+            .GetString();
+    }
+
+    private async Task<bool> IsCoffeeAdvisorEnabledAsync(CancellationToken cancellationToken)
+    {
+        var enabled = await _platform.GetAsync(PlatformIntegrationKeys.OpenAiEnabled, cancellationToken);
+        if (enabled is "false")
+            return false;
+        var feature = await _platform.GetAsync(PlatformIntegrationKeys.OpenAiCoffeeAdvisorEnabled, cancellationToken);
+        return feature is not "false";
+    }
+
+    private async Task<string?> GetApiKeyAsync(CancellationToken cancellationToken)
+    {
+        var fromDb = await _platform.GetAsync(PlatformIntegrationKeys.OpenAiApiKey, cancellationToken);
+        if (!string.IsNullOrWhiteSpace(fromDb))
+            return fromDb.Trim();
+        return _configuration["OpenAI:ApiKey"]?.Trim();
+    }
+
+    private async Task<string> GetModelAsync(CancellationToken cancellationToken)
+    {
+        var fromDb = await _platform.GetAsync(PlatformIntegrationKeys.OpenAiModel, cancellationToken);
+        if (!string.IsNullOrWhiteSpace(fromDb))
+            return fromDb.Trim();
+        return _configuration["OpenAI:Model"]?.Trim() ?? "gpt-4o-mini";
+    }
+}