feat(backend): scaffold product-specific Fastify backend (port 4011)
Add backend/ directory with Fastify 5 + TypeScript ESM service:
- Modules: timers, routines, households, shared-timers, webhooks (migrated from platform-service)
- Cosmos containers: timers, routines, households, shared_timers, webhook_subscriptions, webhook_events
- JWT verification via jose (matches platform-service issuer)
- Shared @bytelyst/* packages via file: refs
- 171 Vitest tests passing
Update AGENTS.md: update backend integration section with product backend details
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