Paddle 收款接入

Adds Paddle Billing (the current Paddle API, not Paddle Classic) to a Next.js App Router app. Covers sandbox setup, Paddle.js overlay checkout tied to the signed-in user, Paddle-Signature verification with timestamp tolerance and secret rotation, event-id dedupe, occurred_at ordering with tombstones, subscription states, scheduled cancels, refunds and chargebacks through adjustments, the customer portal and in-product cancel. Tested against Paddle's official sample payloads. Use when the user wants to sell with Paddle, switch to Paddle as merchant of record, or fix Paddle webhooks, refunds or access.