[FA-10] Adds user service and authentication service

FictionArchive.Service.UserService/Services/UserManagementService.cs

@@ -0,0 +1,45 @@
+using FictionArchive.Service.UserService.Models.Database;
+using Microsoft.EntityFrameworkCore;
+
+namespace FictionArchive.Service.UserService.Services;
+
+public class UserManagementService
+{
+    private readonly ILogger<UserManagementService> _logger;
+    private readonly UserServiceDbContext _dbContext;
+
+    public UserManagementService(UserServiceDbContext dbContext, ILogger<UserManagementService> logger)
+    {
+        _dbContext = dbContext;
+        _logger = logger;
+    }
+
+    public async Task<User> RegisterUser(string username, string email, string oAuthProviderId,
+        string? inviterOAuthProviderId)
+    {
+        var newUser = new User();
+        User? inviter =
+            await _dbContext.Users.FirstOrDefaultAsync(user => user.OAuthProviderId == inviterOAuthProviderId);
+        if (inviter == null && inviterOAuthProviderId != null)
+        {
+            _logger.LogCritical(
+                "A user with OAuthProviderId {OAuthProviderId} was marked as having inviter with OAuthProviderId {inviterOAuthProviderId}, but no user was found with that value.",
+                inviterOAuthProviderId, inviterOAuthProviderId);
+            newUser.Disabled = true;
+        }
+
+        newUser.Username = username;
+        newUser.Email = email;
+        newUser.OAuthProviderId = oAuthProviderId;
+
+        _dbContext.Users.Add(newUser); // Add the new user to the DbContext
+        await _dbContext.SaveChangesAsync(); // Save changes to the database
+        
+        return newUser;
+    }
+
+    public IQueryable<User> GetUsers()
+    {
+        return _dbContext.Users.AsQueryable();
+    }
+}