Add durable gentle nudge engine

[Withphildev]

What changed

• adds transactional migration 9 with durable gentle-nudge state and delivery receipts
• adds timezone-aware nudge preview, status, and execution endpoints
• bundles untimed reminders with cadence by horizon: today every three waking hours, week/overdue daily, month/undated weekly, and longer-range monthly
• adds quiet hours, recent-attention suppression, deterministic delivery IDs, authenticated webhook support, and retry-safe receipts
• fixes calendar-date agenda handling so YYYY-MM-DD stays on the intended local date
• documents configuration and delivery contracts

Why

Nova KanbanX should gently resurface notebook reminders without inventing alert times or creating one cron job per task. Bundling and quiet hours keep the experience persistent but calm and ADHD-friendly.

Impact

The existing UI is unchanged. The API gains /api/nudges/status and /api/nudges/poll. Automatic delivery remains inactive until an authenticated nudge or reminder hook is configured.

Validation

• npm test — 40 tests passed
• npm run build — server and web production builds passed
• npm --workspace web run lint
• git diff --check
• isolated helper/API smoke test with a temporary SQLite database