feat: major implementation - seeder, payments, docs
Seeder API:
- 110 pharmacies across 5 cities (Goiânia 72, Anápolis 22, Nerópolis 10, Senador Canedo 5, Aparecida 1)
- 3-300 products per pharmacy
- Dynamic city/state in company records
Payment Gateways:
- stripe.go: Stripe integration with PaymentIntent
- mock.go: Mock gateway with auto-approve for testing
- PaymentResult and RefundResult domain types
Documentation:
- docs/BACKEND.md: Architecture, invisible fee, endpoints
- docs/SEEDER_API.md: City distribution, product counts
- docs/MARKETPLACE.md: Frontend structure, stores, utils
- docs/BACKOFFICE.md: Admin features, encrypted settings
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