> feat(subscriptions): implement fan-to-creator subscription API

[observerr411]

• Add Subscription entity with fanId, creatorId, status, subscribedAt, cancelledAt
• Unique constraint on fan+creator pair; re-subscribe reactivates existing row
• POST /subscriptions — fan subscribes to creator (409 on duplicate, 400 on self-subscribe)
• DELETE /subscriptions/:creatorId — fan cancels active subscription
• GET /subscriptions/me — fan lists active subscriptions (paginated)
• GET /creators/me/subscribers — creator lists their subscribers (paginated)
• Add migration 1769040000000-CreateSubscriptions
• 14 unit tests + 11 e2e tests, all passing

closes #21

[aji70]

LFG