Merge fix/stripe-billing into development

Stripe billing: rotate to a valid key via Rails credentials; harden error handling; document env vars. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Seto Elkahfi committed Jun 30, 2026 at 22:18 UTC 137199a5368ceef04361a0d437c2663c372a66c2

No changes.