feat(billing): add credit top-ups + billing UI cleanup

[harshithmullapudi]

Summary

• Adds a persistent, non-expiring top-up credit bucket: users on any plan can buy credits in $10 increments (min $10, $10 → 1000 credits) via Stripe Checkout in `payment` mode.
• Adds new `UserUsage.topupCredits` column and `CreditTopup` audit table (migration included). Monthly resets never touch top-up credits.
• Credit spend is now monthly first, then top-up, then overage — atomic via compound optimistic `updateMany`. Applied consistently across `reserveCredits`, `reconcileCredits`, `deductCredits`, `hasCredits`.
• Webhook: new `checkout.session.completed` handler for top-up sessions, idempotent by `topupId` + status.
• UI fixes on `/settings/billing`:
  • New "Add credits" card with $10/$20/$50/$100 preset chips + custom whole-dollar input.
  • Top-up balance surfaces on the Credits card; recent top-ups list.
  • Credits progress bar now fills with % used (was inverted before).
  • Plan card no longer has an empty right side — "View all plans" moved inside.
  • Kills the noisy $0.00 "pending" invoice rows on FREE plans (skip `BillingHistory` when `totalAmount = 0`; also filter legacy rows from the loader).

Files touched

• `packages/database/prisma/schema.prisma` + `migrations/20260703120000_add_credit_topup/`
• `apps/webapp/app/config/billing.server.ts` (topup config + `validateTopupAmount`)
• `apps/webapp/app/services/stripe.server.ts` (`createTopupCheckoutSession`)
• `apps/webapp/app/routes/api.webhooks.stripe.tsx` (checkout webhook + $0 skip)
• `apps/webapp/app/jobs/credit_utils.ts` (reserve/reconcile — 2-bucket)
• `apps/webapp/app/trigger/utils/utils.ts` (deduct/hasCredits/resetMonthly — 2-bucket)
• `apps/webapp/app/services/billing.server.ts` (usage summary + reset)
• `apps/webapp/app/routes/settings.billing.tsx` (UI)

Test plan

• Apply migration (`20260703120000_add_credit_topup`) — verify `UserUsage.topupCredits` and `CreditTopup` table exist.
• Add `checkout.session.completed` to the Stripe webhook endpoint if scoped explicitly.
• On a FREE workspace: click a $10 chip in `/settings/billing`, complete Stripe test-card checkout, verify `CreditTopup.status = completed`, `UserUsage.topupCredits` incremented by 1000, and the "Recent top-ups" list shows the row.
• Drain `availableCredits` to 0 via a few chat/search ops; next op should debit `topupCredits` (not overage). Check that PRO/MAX still overage after both buckets are dry.
• Run the monthly reset trigger against the workspace — confirm `availableCredits` resets to plan quota but `topupCredits` is unchanged.
• Custom-input UX: values below $10 or non-multiples of 10 should be blocked client-side and rejected server-side.
• Verify Invoices list no longer shows $0.00 pending rows (legacy rows hidden, no new ones created).

🤖 Generated with Claude Code