feature(monitor): Scheduled tab UI + cron expression support

[jamby77]

Summary

PR 20 of the MONITOR stack. Adds the Pro+ Scheduled tab on `/monitor` and brings cron support to the scheduler.

Backend

• Nullable `cron_expression` column on `scheduled_captures` (sqlite + postgres + memory). `CHECK` constraints enforce exactly one of `intervalSeconds` or `cronExpression` and the `>=10s` floor stays in place when interval is used
• Idempotent migrations bring PR 19 deployments forward (`PRAGMA table_info` add-column on sqlite, `ADD COLUMN IF NOT EXISTS` on postgres)
• `StoredScheduledCapture` / `ScheduledCapturePatch` carry optional `cronExpression`
• `CaptureScheduler`: picks `CronJob` (via `SchedulerRegistry.addCronJob`) when `cronExpression` is set, `setInterval` otherwise. Distinct prefixes (`monitor-schedule-interval-` / `monitor-schedule-cron-`) keep timer maps clean. Cron expressions are validated server-side via `new CronJob(...)` in a try/throw
• `POST /monitor/schedules` accepts either `intervalSeconds` or `cronExpression` and rejects when both / neither are supplied

Frontend

• New Scheduled tab on `/monitor`, gated by `Feature.MONITOR_SCHEDULED_CAPTURES`
• `SchedulesTable`: created at, status badge, cadence (`every 5m` / `cron: */2 * * * *`), duration, last fire timestamp, last session id, last skip reason, delete action
• `CreateScheduleModal`: amount + unit interval picker by default; Advanced (cron expression) toggle reveals the cron field; capture duration in seconds; success/error states; invalidates the schedules query on save
• `monitorApi.listSchedules / createSchedule / deleteSchedule`

Tests

• `capture-scheduler.spec`: cron register/unregister, validation for missing/conflicting/invalid specs
• `monitor.controller.spec`: BadRequest when both interval and cron are missing; cronExpression forwarding

How to verify

Requires `MONITOR_DEV_PREVIEW=true VITE_MONITOR_DEV_PREVIEW=true` and a Pro+ license loaded.

1. `pnpm dev`. Browser: `/monitor` → click Scheduled tab.
2. Click New schedule → enter `5` and `minutes`, duration `5` → Create. Row appears with cadence `every 5m`. Wait 5 min → `lastFiredAt` populates and a new session shows in the Sessions tab.
3. Open the modal again → toggle Advanced → enter `*/2 * * * *` → Create. Row appears with cadence `cron: */2 * * * *`. Fires on the next even minute.
4. Click Delete → row disappears within one poll.

Test plan

• `pnpm exec tsc --noEmit` (apps/api + apps/web) → clean
• `pnpm exec eslint` on touched files → clean
• `pnpm exec jest --testPathPatterns="capture-scheduler|monitor.controller|storage"` → 151 tests pass
• `pnpm exec vitest run` (apps/web) → 187 tests pass
• Live verify the interval + cron paths against a real Valkey on a Pro+ license