Connect Stripe or GitHub with one key

Registers an app's inbound URL as a webhook at Stripe or GitHub using the provider's own key, saves the signing secret the provider yields, and turns on signature verification at the edge. The provider is read from the key prefix (sk_/rk_ for Stripe, github_pat_/ghp_ for GitHub). The key is used for these calls only and is not stored. Without app, a new app is created for the provider (and removed again if the provider refuses the key). Re-running replaces the registration instead of duplicating it. Least privilege: a Stripe restricted key with Webhook Endpoints = Write, or a fine-grained GitHub token with Webhooks = Read and write.