feat(billing): add Stripe sandbox verification(Phase 6 Stripe Sandbox)

[law-chain-hot]

Summary

• wire stage-scoped Stripe API and webhook secrets into the API service
• add a guarded Stripe CLI listener that checks test mode, account identity, webhook secret, and consumed event types
• add a real browser + PostgreSQL Stripe Sandbox E2E for payment setup and a $5 top-up
• clarify saved-card consent before redirecting to Stripe

Verification

• yarn test:billing:verify
• yarn e2e:billing:stripe
• 63 API tests, 9 Stripe config tests, 7 PostgreSQL edge tests, 39 Dashboard tests
• API and Dashboard production builds
• real Stripe result: +500 cents and exactly one wallet ledger credit

Scope

• no database schema changes
• reconcile, refund/dispute handling, and operational recovery remain in the next PR

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/billing-v3-pr6-stripe-sandbox

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