chore: add deploy-subscription-fpc script and graceful FPC handling

- Add scripts/deploy-subscription-fpc.ts for deploying SubscriptionFPC to local
- Make registerDripContracts skip gracefully when SubscriptionFPC is not on-chain
- Add design spec and implementation plan docs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mverzilli