backend phase 0: foundation, cross-cutting seams & starter cleanup

Remove the Order demo (entity/feature/repo/config/gRPC/proto) and the three
pre-marketplace migrations; regenerate a fresh InitialBaseline migration.

Stand up the REST surface (PingController + System/Ping CQRS) proving the
Mediator -> behaviors -> OperationResult -> ApiResult envelope end to end.

Close wiring gaps: register LoggingBehavior (outermost) and add the built-in
rate limiter (per-IP global + otp/auth/sensitive policies), placed before
authentication.

Add current-user + audit plumbing: ICurrentUser (HttpContext + null impls),
rename BaseEntity audit fields to CreatedAt/ModifiedAt (DateTimeOffset) +
CreatedById/ModifiedById, stamped by a new AuditFieldInterceptor.

Introduce five cross-cutting seams (IDateTimeProvider, IFieldEncryptor,
ICacheService, IObjectStorage, INotificationDispatcher) with in-memory/local
mocks registered via AddCrossCuttingSeams.

Add Baya.Test.Foundation (encryptor, audit interceptor, ping handler) and
update docs, contracts (swagger.v1.json), handoff, report, and mocks registry.

server/src/Infrastructure/Baya.Infrastructure.CrossCutting/Seams/SeamOptions.cs

@@ -0,0 +1,28 @@
+namespace Baya.Infrastructure.CrossCutting.Seams;
+
+/// <summary>
+/// Options bound from the <c>Seams</c> configuration section. The mock seams read non-secret defaults
+/// from here; production keys/paths come from environment variables or user-secrets, never committed.
+/// </summary>
+public sealed class SeamOptions
+{
+    public const string SectionName = "Seams";
+
+    public FieldEncryptionOptions FieldEncryption { get; set; } = new();
+    public ObjectStorageOptions ObjectStorage { get; set; } = new();
+}
+
+public sealed class FieldEncryptionOptions
+{
+    /// <summary>Base64 32-byte AES key. Local-dev default only; override per environment.</summary>
+    public string Key { get; set; } = string.Empty;
+
+    /// <summary>Base64 HMAC key used for deterministic lookup hashes.</summary>
+    public string HashKey { get; set; } = string.Empty;
+}
+
+public sealed class ObjectStorageOptions
+{
+    /// <summary>Filesystem root the local-disk mock writes blobs under.</summary>
+    public string RootPath { get; set; } = string.Empty;
+}