feat(manager-core): admin auth gate (Phase 3a)

Closes the regression risk of the admin API and dashboard being open
to anyone reaching the bound port. Required foundation before v1.1
data-plane services land.

Per-user accounts (admin_users), Argon2id passwords, env-var bootstrap
of the first admin that becomes inert once any admin exists, opaque
32-byte session token doubling as bearer credential, 24h sliding TTL
configurable via PICLOUD_SESSION_TTL_HOURS. is_active column lets
admins be deactivated without losing audit history; last-active-admin
guard on DELETE and on PATCH that flips is_active to false (sessions
also wiped on deactivation).

require_admin middleware fronts every /api/v1/admin/* route. The data
plane (/api/v1/execute/{id}), /healthz, /version, and user routes
stay open. picloud admin reset-password <username> subcommand handles
recovery without going through HTTP.

Dashboard gains /admin/login and /admin/admins surfaces, a top-bar
user menu, and a token store with a localStorage echo so refreshes
don't sign you out. Cookie-based auth works in parallel for non-SPA
clients.

Forward compatibility: future RBAC tables (admin_roles,
admin_user_roles) join on admin_users.id; the auth middleware is the
seam where role checks slot in. Email, 2FA, passkeys, and personal
API tokens are all additive without touching admin_users.

Blueprint §11.4 updated to reflect what actually shipped.

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

crates/manager-core/src/lib.rs

@@ -4,7 +4,14 @@
 //! the same DB for now; once we add caching and per-node ingress, the
 //! manager will publish change events.
 
+pub mod admin_session_repo;
+pub mod admin_user_repo;
+pub mod admin_users_api;
 pub mod api;
+pub mod auth;
+pub mod auth_api;
+pub mod auth_bootstrap;
+pub mod auth_middleware;
 pub mod log_sink;
 pub mod migrations;
 pub mod repo;
@@ -13,7 +20,21 @@ pub mod route_repo;
 pub mod sandbox;
 pub mod scheduler;
 
+pub use admin_session_repo::{
+    AdminSessionLookup, AdminSessionRepository, AdminSessionRepositoryError,
+    PostgresAdminSessionRepository,
+};
+pub use admin_user_repo::{
+    AdminUserCredentials, AdminUserRepository, AdminUserRepositoryError, AdminUserRow,
+    PostgresAdminUserRepository,
+};
+pub use admin_users_api::{admins_router, AdminsState};
 pub use api::{admin_router, AdminState};
+pub use auth_api::auth_router;
+pub use auth_bootstrap::{
+    bootstrap_first_admin, bootstrap_first_admin_with, BootstrapEnv, BootstrapError,
+};
+pub use auth_middleware::{require_admin, AuthState, AuthedAdmin, SESSION_COOKIE};
 pub use log_sink::PostgresExecutionLogSink;
 pub use repo::{
     ExecutionLogRepository, NewScript, PostgresExecutionLogRepository, PostgresScriptRepository,