feat(shared): add Principal, InstanceRole, AppRole, Scope, ApiKeyId

Cross-crate authn/authz data types for Phase 3.5. The Principal struct
is the resolved caller identity that auth_middleware will produce for
both cookie sessions and bearer API keys; the role/scope enums mirror
the DB CHECK constraints from migration 0006 and round-trip through
their stable string forms.

UserId is a type alias for AdminUserId — the auth layer treats an
admin row as the principal identity, so the alias avoids a rename of
the existing id type.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/shared/src/lib.rs

@@ -5,6 +5,7 @@
 //! entity, error roots, transport DTOs).
 
 pub mod app;
+pub mod auth;
 pub mod error;
 pub mod execution_log;
 pub mod ids;
@@ -16,9 +17,10 @@ pub mod validator;
 pub mod version;
 
 pub use app::{App, AppDomain, DomainShape};
+pub use auth::{AppRole, InstanceRole, Principal, Scope, UserId};
 pub use error::Error;
 pub use execution_log::{ExecutionLog, ExecutionStatus};
-pub use ids::{AdminUserId, AppId, ExecutionId, RequestId, ScriptId};
+pub use ids::{AdminUserId, ApiKeyId, AppId, ExecutionId, RequestId, ScriptId};
 pub use log_sink::{ExecutionLogSink, LogSinkError};
 pub use route::{HostKind, PathKind, Route};
 pub use sandbox::ScriptSandbox;