refactor(platform-service): replace PRODUCT_ID with getRequestProductId(req) in all modules
- 26 files updated: all repositories accept productId parameter instead of env var
- All route handlers extract productId via getRequestProductId(req) (JWT → header → env fallback)
- Repositories: auth, flags, audit, notifications, licenses, plans, referrals, usage, subscriptions, invitations
- Routes: all above + promos, items, memory, public, ratelimit, stripe
- lib/webhooks.ts: dispatchWebhook accepts optional productId parameter
- Stripe webhook handler uses metadata-based productId (no client JWT available)
- Ratelimit default config uses DEFAULT_PRODUCT_ID at startup
- 166 tests pass, tsc --noEmit clean
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