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>

frontend/src/pages/JobsPage.test.ts

@@ -5,7 +5,7 @@ import { useAuthStore } from '../stores/auth'
 import { api } from '../api/client'
 
 vi.mock('../api/client', () => ({
-  api: { get: vi.fn(), post: vi.fn(), patch: vi.fn() },
+  api: { get: vi.fn(), post: vi.fn(), patch: vi.fn(), put: vi.fn() },
   ApiError: class extends Error {},
 }))