BRAKING refactor project structure.

Refactor: the project is now divided into a more clear structure, with **Infrastructure** and **Application** layers added.

Refactor: configurations are split into sections for different layers.

Fix: now EF Core related operations, such as migration, should be invoked in `OptixServe.Infrastructure`, with config file and data dir passed into `dotnet ef` command. See `OptixServe.Infrastructure/Utilites/DesignTimeDbContextFactory.cs` for details.

Fix: EF migrations are ignored in gitignore on purpose in early development.

OptixServe.Application/OptixServe.Application.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <ItemGroup>
+    <ProjectReference Include="..\OptixServe.Core\OptixServe.Core.csproj" />
+    <ProjectReference Include="..\OptixServe.Infrastructure\OptixServe.Infrastructure.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="9.0.6" />
+  </ItemGroup>
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

OptixServe.Application/Services/UserService.cs

@@ -0,0 +1,31 @@
+using OptixServe.Core.Models;
+using OptixServe.Infrastructure.Data;
+
+namespace OptixServe.Application.Services;
+
+public interface IUserService
+{
+    IEnumerable<User> GetUsers();
+    User? GetUserById(string id);
+    User? GetUserByUsername(string username);
+}
+
+public class UserService(AppDbContext dbContext) : IUserService
+{
+    private readonly AppDbContext _dbContext = dbContext;
+
+    public User? GetUserById(string id)
+    {
+        return _dbContext.Users.FirstOrDefault(u => u.Id == id);
+    }
+
+    public User? GetUserByUsername(string username)
+    {
+        return _dbContext.Users.FirstOrDefault(u => u.UserName == username);
+    }
+
+    public IEnumerable<User> GetUsers()
+    {
+        return _dbContext.Users.AsEnumerable();
+    }
+}