Document subscription readiness architecture

[biru-codeastromer]

What changed

• document a provider-neutral billing and entitlement architecture
• cover weekly, monthly, yearly, and lifetime INR offers without choosing prices
• define durable signed webhooks, idempotency, reconciliation, refunds, grace periods, and provider migration
• record India-first UPI AutoPay, invoice, tax-review, security, testing, operations, and activation requirements
• keep billing, checkout, provider secrets, and paid entitlement enforcement explicitly disabled

Why

Katalume needs a safe blueprint before pricing or a payment provider is selected. This gives product and engineering one implementation contract without adding dormant payment code or exposing a checkout before legal, tax, support, and production gates are ready.

Checks

• mkdocs build --strict
• git diff --check