Updates

Initial features: payments & balance, subscriptions via PayPal/Stripe, admin tools.

• My Payments page: KP balance, coin purchase, Basic/Standard/Pro subscriptions
• Subscriptions on Stripe/PayPal with SKU-based resolution and env toggles
• Admin tools: PayPal catalog create/list, Stripe products/prices with ID copy
• Unified billing design (billing.md): webhooks as SoT, idempotent upserts, audit logs
• Cloudflare compatibility: fetch-based legal docs, CI lockfile workaround