feat(wbs): AEG-VS-01-04 BE Vertical Slice - Endpoints & Handlers (Part 1)

- RegisterIdentity endpoint (POST /api/identities)
- RequestMfaSetup endpoint (PUT /api/identities/{id}/request-mfa)
- SQL repositories w/ optimistic concurrency (revision tracking)
- Application handlers (IEndpointHandler pattern)
- ValidationException + ProblemDetails error handling
- Unit tests: RegisterIdentityHandlerTests (4), RequestMfaSetupHandlerTests (4)
- Domain state machines integrated (IdentityState lifecycle)
- AGENTS.md v16.0: endpoint authority, idempotency, correlation ID ready

DI registration & integration tests deferred to next session.
17 new files, 500+ LOC, 8/8 unit tests ready to run

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

src/Modules/IdentityAccess/ManageIdentityAndRoles/Features/RegisterIdentity/RegisterIdentitySql.cs

@@ -0,0 +1,68 @@
+using Dapper;
+using Npgsql;
+
+namespace KArtSell.Modules.IdentityAccess.ManageIdentityAndRoles.Features.RegisterIdentity;
+
+public interface IRegisterIdentitySql
+{
+    Task<bool> EmailExistsAsync(string email, CancellationToken ct);
+    Task<Guid> CreateIdentityAsync(Guid id, string email, string displayName, string correlationId, CancellationToken ct);
+    Task<(Guid Id, string Email, string DisplayName, string State)> GetIdentityAsync(Guid id, CancellationToken ct);
+}
+
+public sealed class RegisterIdentitySql : IRegisterIdentitySql
+{
+    private readonly Func<Task<NpgsqlConnection>> _connectionFactory;
+
+    public RegisterIdentitySql(Func<Task<NpgsqlConnection>> connectionFactory)
+    {
+        _connectionFactory = connectionFactory;
+    }
+
+    public async Task<bool> EmailExistsAsync(string email, CancellationToken ct)
+    {
+        using var conn = await _connectionFactory();
+        const string sql = """
+            SELECT EXISTS(SELECT 1 FROM identity.identity WHERE email = @email)
+            """;
+        return await conn.QuerySingleAsync<bool>(sql, new { email }, commandTimeout: 5);
+    }
+
+    public async Task<Guid> CreateIdentityAsync(Guid id, string email, string displayName, string correlationId, CancellationToken ct)
+    {
+        using var conn = await _connectionFactory();
+        const string sql = """
+            INSERT INTO identity.identity (id, email, display_name, state, created_at, updated_at, published_at, revision_version, correlation_id)
+            VALUES (@id, @email, @displayName, @state, NOW(), NOW(), NOW(), 1, @correlationId)
+            ON CONFLICT (email) DO NOTHING
+            RETURNING id;
+            """;
+
+        var result = await conn.QuerySingleOrDefaultAsync<Guid?>(sql, new
+        {
+            id,
+            email,
+            displayName,
+            state = Domain.IdentityState.Active,
+            correlationId
+        }, commandTimeout: 5);
+
+        return result ?? Guid.Empty;
+    }
+
+    public async Task<(Guid Id, string Email, string DisplayName, string State)> GetIdentityAsync(Guid id, CancellationToken ct)
+    {
+        using var conn = await _connectionFactory();
+        const string sql = """
+            SELECT id, email, display_name, state
+            FROM identity.identity
+            WHERE id = @id
+            """;
+
+        var row = await conn.QuerySingleOrDefaultAsync<dynamic>(sql, new { id }, commandTimeout: 5);
+        if (row is null)
+            throw new InvalidOperationException($"Identity {id} not found");
+
+        return ((Guid)row.id, (string)row.email, (string)row.display_name, (string)row.state);
+    }
+}