feat(notifications): D1 data model + idempotency guard

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Parent roadmap: #525
Parent epic: #535

Goal

D1 data model: notification_subscriptions + notification_log with a UNIQUE(dedup_key, channel) idempotency guard and a per-recipient rate-limit window.

Reuse

Drizzle patterns mirroring digest_subscriptions (src/db/schema.ts).

Acceptance

Migration applies; duplicate dedup_key insert is a no-op.

Testing

npm run test:ci, 97%+ coverage. Public/private output boundary preserved; deterministic where applicable.

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Verified shipped (2026-06-17 audit): notifications D1 data model + idempotency guard present and exercised by notifications-service.test.ts / notifications-events.test.ts. Closing (email channel remains in #570).