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/RequestMfaSetup/RequestMfaSetupSql.cs

@@ -0,0 +1,59 @@
+using Dapper;
+using Npgsql;
+
+namespace KArtSell.Modules.IdentityAccess.ManageIdentityAndRoles.Features.RequestMfaSetup;
+
+public interface IRequestMfaSetupSql
+{
+    Task<(Guid Id, string State, int Revision)> GetIdentityAsync(Guid identityId, CancellationToken ct);
+    Task UpdateIdentityStateAsync(Guid identityId, string newState, int expectedRevision, CancellationToken ct);
+}
+
+public sealed class RequestMfaSetupSql : IRequestMfaSetupSql
+{
+    private readonly Func<Task<NpgsqlConnection>> _connectionFactory;
+
+    public RequestMfaSetupSql(Func<Task<NpgsqlConnection>> connectionFactory)
+    {
+        _connectionFactory = connectionFactory;
+    }
+
+    public async Task<(Guid Id, string State, int Revision)> GetIdentityAsync(Guid identityId, CancellationToken ct)
+    {
+        using var conn = await _connectionFactory();
+        const string sql = """
+            SELECT id, state, revision_version
+            FROM identity.identity
+            WHERE id = @identityId
+            """;
+
+        var row = await conn.QuerySingleOrDefaultAsync<dynamic>(sql, new { identityId }, commandTimeout: 5);
+        if (row is null)
+            throw new InvalidOperationException($"Identity {identityId} not found");
+
+        return ((Guid)row.id, (string)row.state, (int)row.revision_version);
+    }
+
+    public async Task UpdateIdentityStateAsync(Guid identityId, string newState, int expectedRevision, CancellationToken ct)
+    {
+        using var conn = await _connectionFactory();
+        const string sql = """
+            UPDATE identity.identity
+            SET state = @newState,
+                revision_version = revision_version + 1,
+                updated_at = NOW(),
+                published_at = NOW()
+            WHERE id = @identityId AND revision_version = @expectedRevision
+            """;
+
+        var rowsAffected = await conn.ExecuteAsync(sql, new
+        {
+            identityId,
+            newState,
+            expectedRevision
+        }, commandTimeout: 5);
+
+        if (rowsAffected == 0)
+            throw new InvalidOperationException("Optimistic concurrency violation: state changed");
+    }
+}