Harden gentle nudge delivery

[Withphildev]

What changed

• extracted the gentle-nudge planner, delivery engine, receipts, and retry guard from server/src/index.ts into a dedicated service
• made KANBAN_NUDGE_TIMEZONE the default for API and background polling
• added migration 10 to persist the complete first-attempt nudge payload
• made retries reuse the exact stored JSON payload and idempotency key even when cards change between attempts
• documented HTTPS requirements for hook bearer tokens

Why

The previous API routes defaulted to America/Los_Angeles even when a different automatic nudge timezone was configured. Retry receipts also froze only card IDs and the message while regenerating card details, so the delivered payload could drift from the durable receipt after an edit.

Impact

Existing successful receipts are unchanged. New nudge attempts store a durable payload snapshot. Existing failed receipts remain retryable and are snapshotted on their next attempt. The UI is unchanged.

Validation

• npm test — 41 tests passed
• npm run build — server and web production builds passed