WP-02: encrypted settings, SMTP mail, job runner and cron scheduler

AES-256-GCM secret storage keyed by MASTER_KEY, SMTP settings with test mail
(lettre), persisted job runs with log and status, JobRunner with per-kind
concurrency guard, 6-field cron schedules with defaults, scheduler loop.
Settings and Jobs pages in the UI. FAKE_HOST mode for dev machines.

Tests: 30 application, 8 infrastructure, 23 API, 16 Vitest, 6 Playwright.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/crates/api/Cargo.toml

@@ -13,6 +13,7 @@ domain.workspace = true
 application.workspace = true
 infrastructure.workspace = true
 anyhow.workspace = true
+async-trait.workspace = true
 axum.workspace = true
 chrono.workspace = true
 dotenvy.workspace = true