add architecture skeleton

GymLogAI.AI/DependencyInjection.cs

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+using GymLogAI.Application.Interfaces;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace GymLogAI.AI;
+
+public static class DependencyInjection
+{
+    public static IServiceCollection AddAiServices(this IServiceCollection services, IConfiguration configuration)
+    {
+        services
+            .AddOptions<OpenRouterOptions>()
+            .Bind(configuration.GetSection(OpenRouterOptions.SectionName));
+
+        services.AddHttpClient<OpenRouterChatClient>((serviceProvider, client) =>
+        {
+            var options = serviceProvider.GetRequiredService<Microsoft.Extensions.Options.IOptions<OpenRouterOptions>>().Value;
+            client.BaseAddress = new Uri(options.BaseUrl);
+        });
+
+        services.AddHttpClient<OpenRouterEmbeddingClient>((serviceProvider, client) =>
+        {
+            var options = serviceProvider.GetRequiredService<Microsoft.Extensions.Options.IOptions<OpenRouterOptions>>().Value;
+            client.BaseAddress = new Uri(options.BaseUrl);
+        });
+
+        services.AddScoped<IWorkoutParser, WorkoutParserAIService>();
+        services.AddScoped<IWorkoutRecommendationGenerator, WorkoutRecommendationAIService>();
+        services.AddScoped<IEmbeddingService, StubEmbeddingService>();
+
+        return services;
+    }
+}

GymLogAI.AI/GymLogAI.AI.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Http" Version="10.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" Version="10.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\GymLogAI.Application\GymLogAI.Application.csproj" />
+    <ProjectReference Include="..\GymLogAI.Core\GymLogAI.Core.csproj" />
+  </ItemGroup>
+
+</Project>

GymLogAI.AI/OpenRouterChatClient.cs

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+using System.Net.Http.Json;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace GymLogAI.AI;
+
+public sealed class OpenRouterChatClient(
+    HttpClient httpClient,
+    IOptions<OpenRouterOptions> options,
+    ILogger<OpenRouterChatClient> logger)
+{
+    private readonly OpenRouterOptions _options = options.Value;
+
+    public async Task<string?> CompleteAsync(string prompt, CancellationToken cancellationToken)
+    {
+        if (string.IsNullOrWhiteSpace(_options.ApiKey))
+        {
+            logger.LogInformation("OpenRouter API key is not configured. Returning stubbed chat completion.");
+            return null;
+        }
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, "chat/completions")
+        {
+            Content = JsonContent.Create(new
+            {
+                model = _options.ChatModel,
+                messages = new[] { new { role = "user", content = prompt } }
+            })
+        };
+
+        request.Headers.Authorization = new("Bearer", _options.ApiKey);
+        var response = await httpClient.SendAsync(request, cancellationToken);
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadAsStringAsync(cancellationToken);
+    }
+}

GymLogAI.AI/OpenRouterEmbeddingClient.cs

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+using System.Net.Http.Json;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace GymLogAI.AI;
+
+public sealed class OpenRouterEmbeddingClient(
+    HttpClient httpClient,
+    IOptions<OpenRouterOptions> options,
+    ILogger<OpenRouterEmbeddingClient> logger)
+{
+    private readonly OpenRouterOptions _options = options.Value;
+
+    public async Task<float[]?> GenerateAsync(string input, CancellationToken cancellationToken)
+    {
+        if (string.IsNullOrWhiteSpace(_options.ApiKey))
+        {
+            logger.LogInformation("OpenRouter API key is not configured. Returning stubbed embedding.");
+            return null;
+        }
+
+        using var request = new HttpRequestMessage(HttpMethod.Post, "embeddings")
+        {
+            Content = JsonContent.Create(new
+            {
+                model = _options.EmbeddingModel,
+                input
+            })
+        };
+
+        request.Headers.Authorization = new("Bearer", _options.ApiKey);
+        var response = await httpClient.SendAsync(request, cancellationToken);
+        response.EnsureSuccessStatusCode();
+
+        return null;
+    }
+}

GymLogAI.AI/OpenRouterOptions.cs

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+namespace GymLogAI.AI;
+
+public sealed class OpenRouterOptions
+{
+    public const string SectionName = "OpenRouter";
+
+    public string BaseUrl { get; init; } = "https://openrouter.ai/api/v1/";
+    public string? ApiKey { get; init; }
+    public string ChatModel { get; init; } = "openai/gpt-4.1-mini";
+    public string EmbeddingModel { get; init; } = "text-embedding-3-small";
+}

GymLogAI.AI/StubEmbeddingService.cs

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+using GymLogAI.Application.Interfaces;
+
+namespace GymLogAI.AI;
+
+public sealed class StubEmbeddingService(OpenRouterEmbeddingClient embeddingClient) : IEmbeddingService
+{
+    public async Task<float[]> GenerateEmbeddingAsync(string input, CancellationToken cancellationToken)
+    {
+        var remoteEmbedding = await embeddingClient.GenerateAsync(input, cancellationToken);
+        if (remoteEmbedding is not null)
+        {
+            return remoteEmbedding;
+        }
+
+        return
+        [
+            input.Length,
+            input.Count(char.IsLetter),
+            input.Count(char.IsDigit),
+            input.Count(char.IsWhiteSpace)
+        ];
+    }
+}

GymLogAI.AI/WorkoutParserAIService.cs

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+using GymLogAI.Application.DTOs;
+using GymLogAI.Application.Interfaces;
+
+namespace GymLogAI.AI;
+
+public sealed class WorkoutParserAIService(OpenRouterChatClient chatClient) : IWorkoutParser
+{
+    public async Task<ParsedWorkoutDto?> ParseAsync(string text, CancellationToken cancellationToken)
+    {
+        await chatClient.CompleteAsync($"Parse workout:\n{text}", cancellationToken);
+
+        var lines = text
+            .Split(['\r', '\n'], StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Where(x => !string.IsNullOrWhiteSpace(x))
+            .ToArray();
+
+        if (lines.Length == 0)
+        {
+            return null;
+        }
+
+        var exercises = lines
+            .Select((line, index) => new ParsedExerciseDto(
+                line,
+                index + 1,
+                null,
+                [new ParsedExerciseSetDto(1, 10, null, null, null, "Stub set")]))
+            .ToArray();
+
+        return new ParsedWorkoutDto(
+            DateTime.UtcNow,
+            "Imported workout",
+            "Stubbed parser result",
+            exercises);
+    }
+}

GymLogAI.AI/WorkoutRecommendationAIService.cs

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+using GymLogAI.Application.DTOs;
+using GymLogAI.Application.Interfaces;
+using GymLogAI.Core.Entities;
+
+namespace GymLogAI.AI;
+
+public sealed class WorkoutRecommendationAIService(OpenRouterChatClient chatClient) : IWorkoutRecommendationGenerator
+{
+    public async Task<WorkoutRecommendationDto> GenerateAsync(
+        WorkoutRecommendationContext context,
+        IReadOnlyCollection<Workout> workoutHistory,
+        CancellationToken cancellationToken)
+    {
+        await chatClient.CompleteAsync(
+            $"Recommend workout for goal {context.GoalType} with {workoutHistory.Count} historical workouts.",
+            cancellationToken);
+
+        var lastWorkout = workoutHistory.FirstOrDefault();
+        var fallbackExercises = lastWorkout?.WorkoutExercises
+            .OrderBy(x => x.Order)
+            .Select(x => x.Exercise?.Name ?? "Bodyweight movement")
+            .Distinct()
+            .Take(4)
+            .ToArray();
+
+        return new WorkoutRecommendationDto(
+            "Today's training recommendation",
+            $"Goal: {context.GoalType}. History count: {workoutHistory.Count}.",
+            fallbackExercises is { Length: > 0 }
+                ? fallbackExercises
+                : ["Squat", "Bench Press", "Row", "Plank"],
+            "Keep the session moderate and leave 1-2 reps in reserve.");
+    }
+}