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>

.env.example

@@ -1,6 +1,10 @@
 # Backend configuration (copy to backend/.env for local development)
 DATABASE_URL=sqlite://data/monitoring.db?mode=rwc
 JWT_SECRET=change-me-to-a-long-random-string
+# 64 hex characters (openssl rand -hex 32); encrypts stored secrets such as SMTP passwords
+MASTER_KEY=change-me-64-hex-characters
+# true on dev machines without apt/kubectl: uses fake host adapters with sample data
+FAKE_HOST=false
 BIND=127.0.0.1:8080
 # Created on first start if no user exists
 BOOTSTRAP_ADMIN_EMAIL=admin@example.com