fix(team-members): map duplicate-email race to conflict error (SH-325)

CreateAsync checks FindByEmailAsync and then inserts, so two concurrent
creates with the same email can both pass the check and hit the unique
user-name index. That DbUpdateException was unhandled and surfaced as a
500. Catch it at the CreateAsync call and return the existing
"Email is already in use." message, matching the check-then-insert
converge pattern already used by SetOverrideCoreAsync.

Api.SeaHavenIndustries.Tests/TeamMemberServiceTests.cs

@@ -3,6 +3,7 @@ using Data.SeaHavenIndustries;
 using Data.SeaHavenIndustries.Enums;
 using FluentAssertions;
 using Microsoft.AspNetCore.Identity;
+using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using Moq;
@@ -95,6 +96,26 @@ public sealed class TeamMemberServiceTests
         overrides.Verify(data => data.SetOverridesAsync("new-user", It.Is<IReadOnlyDictionary<string, UserPermissionState>>(value => value["deleteSites"] == UserPermissionState.Allow), It.IsAny<CancellationToken>()), Times.Once);
     }
 
+    [Fact]
+    public async Task Create_ConcurrentDuplicateEmail_ReturnsAlreadyInUseInsteadOf500()
+    {
+        var service = NewService(out var userManager, out var roleManager, out _, out _);
+        userManager
+            .Setup(manager => manager.FindByEmailAsync(It.IsAny<string>()))
+            .ReturnsAsync((ApplicationUser?)null);
+        roleManager.Setup(manager => manager.RoleExistsAsync("Dispatcher")).ReturnsAsync(true);
+        userManager
+            .Setup(manager => manager.CreateAsync(It.IsAny<ApplicationUser>()))
+            .ThrowsAsync(new DbUpdateException("duplicate key", new Exception()));
+
+        var result = await service.CreateAsync(ValidRequest(), Admin(), CancellationToken.None);
+
+        result.Success.Should().BeFalse();
+        result.Error.Should().Be("Email is already in use.");
+        userManager.Verify(manager => manager.AddToRoleAsync(It.IsAny<ApplicationUser>(), It.IsAny<string>()), Times.Never);
+        userManager.Verify(manager => manager.DeleteAsync(It.IsAny<ApplicationUser>()), Times.Never);
+    }
+
     private static TeamMemberService NewService(
         out Mock<UserManager<ApplicationUser>> userManager,
         out Mock<RoleManager<IdentityRole>> roleManager,

SeaHaven.Services/Implementation/TeamMemberService.cs

@@ -3,6 +3,7 @@ using System.Security.Claims;
 using Data.SeaHavenIndustries;
 using Data.SeaHavenIndustries.Enums;
 using Microsoft.AspNetCore.Identity;
+using Microsoft.EntityFrameworkCore;
 using SeaHaven.DataServices.Interfaces;
 using SeaHaven.Services.Constants;
 using SeaHaven.Services.DTOs;
@@ -68,7 +69,20 @@ public sealed class TeamMemberService : ITeamMemberService
             PendingRegistrationCreatedDate = now
         };
 
-        var createResult = await _userManager.CreateAsync(user);
+        IdentityResult createResult;
+        try
+        {
+            createResult = await _userManager.CreateAsync(user);
+        }
+        catch (DbUpdateException)
+        {
+            // A concurrent create won the race on the unique user-name index
+            // between the FindByEmailAsync check above and this insert. Surface
+            // the same conflict message instead of letting the database
+            // exception bubble up as a 500.
+            return Failure("Email is already in use.");
+        }
+
         if (!createResult.Succeeded)
             return Failure(createResult.Errors.FirstOrDefault()?.Description ?? "Unable to create team member.");