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/infrastructure/migrations/0002_settings_and_jobs.sql

@@ -0,0 +1,16 @@
+CREATE TABLE settings (
+    key   TEXT PRIMARY KEY,
+    value TEXT NOT NULL
+);
+
+CREATE TABLE job_runs (
+    id           TEXT PRIMARY KEY,
+    kind         TEXT NOT NULL,
+    params       TEXT,
+    status       TEXT NOT NULL CHECK (status IN ('running', 'success', 'failed')),
+    started_at   TEXT NOT NULL,
+    finished_at  TEXT,
+    log          TEXT NOT NULL DEFAULT '',
+    triggered_by TEXT NOT NULL
+);
+CREATE INDEX job_runs_kind_started ON job_runs(kind, started_at DESC);