Begin Stripe Connect OAuth (Quickstart mode)

Generates a Stripe Connect authorize URL and a signed state token. The frontend redirects the user's browser to authorizeUrl; Stripe then redirects back to /v1/integrations/stripe/oauth/callback where Drip exchanges the code, creates the integration, and auto-provisions the webhook endpoint. Gated on the STRIPE_QUICKSTART_ENABLED feature flag; returns 404 if disabled for the workspace.