[US-051] Subscribe/upgrade/downgrade plan via payment gateway

[leimo2011]

Epic: EP-09 — Subscription & Billing \nPriority: Must Have (M) \nStory Points: 8 \nSprint: Sprint 4\n\n### User Story\nAs a store admin, I want to subscribe/upgrade/downgrade my plan via a payment gateway so billing is seamless.\n\n### Acceptance Criteria\n- [ ] Plan selection screen with feature comparison table\n- [ ] Stripe Checkout integration for initial subscription\n- [ ] Upgrade/downgrade proration handled correctly\n- [ ] Subscription status reflected immediately in DB\n- [ ] Admin receives email receipt after payment\n- [ ] Failed payment triggers grace period notification\n\n### Technical Notes\n- Stripe webhooks: customer.subscription.updated, invoice.payment_failed\n- POST /api/subscriptions/checkout/ — create Stripe session\n- subscription_plans + subscriptions tables\n- PayMongo (PH) added in Sprint 5