billing: $10 signup credit + lazy claim modal; disable welcome credit promo (#674)

* billing: $10 signup credit + lazy claim modal; disable welcome credit promo

Adds a per-Account $10 Router signup credit granted on first Router-tab
mount via a new admin-gated POST /api/account/[owner]/signup-credit/claim.
The endpoint is idempotent — the inserted CreditGrant row IS the dedup
state, so subsequent calls return granted:false. Client SignupCreditModal
fires the POST on mount (only when modelAccessMode === "router") and
opens a celebratory dialog when granted:true.

Disables the legacy welcome credit ($10 on first card add) via a new
WELCOME_CREDIT_PROMO_ACTIVE = false flag in utils/stripe.ts. Code path
stays intact — flip the flag to revive. Strips the now-untruthful
"$10 on enabling billing" copy from BillingCard, EnableRouterPrompt,
triggerWorkflow paywall comment, action router_requires_card summary,
email snippet, billing/pricing docs and wiki.

Cuts WELCOME_CREDIT_CENTS from 2000 to 1000 to reflect the lower amount
that would land if the flag is ever re-enabled. Adds "signup" reason
mapping to BillingCard wallet history.

Verified end-to-end against dev: admin+Router fires modal, admin+BYOK
gate-blocks mount, BYOK→Router transition fires modal on click, member
and collaborator paths skip the mount entirely, reload after grant is
idempotent. Wallet history shows "Router signup credit +$10.00".

* billing: address PR review (race fix, copy sweep, modal retry)

Correctness:
- Add @@unique([accountId, reason]) on CreditGrant + migration. The prior
  check-then-insert pattern in /signup-credit/claim and finalizeCheckoutSession
  raced at READ COMMITTED — two concurrent admin tabs could land two grants of
  the same reason on a fresh account ($10 each). Both write sites now rely on
  the unique index for dedup (P2002 = "already granted") and route updated to
  catch P2002 cleanly. Verified zero existing duplicates in prod before
  migration.
- Add log.info on signup grant insert so a successful grant has any chance of
  being caught by ops monitoring.
- Add retry: 2 with backoff to the claim mutation. Endpoint is idempotent so
  a server-side success that lost its response cleanly returns granted:false
  on retry.

Public copy that still advertised the (now-deleted) $20 welcome credit:
- app/page.tsx landing pricing card
- emails/announceBilling.ts broadcast template
- docs/keys.mdx BYOK note
- components/AgentSettings.tsx Router-without-billing warning
- utils/stripe.ts finalizeCheckoutSession JSDoc
- utils/email/snippets.ts ROUTER_CREDIT_PS_HTML JSDoc

Wiki staleness sweep:
- wiki/billing.md TOC, mermaid diagram (signup edge added; welcome marked
  dormant), test coverage list, key modules section, no-card wallet narrative
- wiki/pricing.md welcome-credit drawdown reference
- Rewrote my own internally-inconsistent dormancy paragraph to be honest
  about the $20-historical / $10-on-revival framing.

Trivia:
- ModelAccessCard JSX comment had a literal \\u2192 instead of →.

* billing: address PR review round 2

- Replace try/catch P2002 inside finalizeCheckoutSession's prisma.$transaction
  with createMany skipDuplicates. The previous form is broken on Postgres: a
  unique-violation poisons the surrounding TX, so the catch block returns
  cleanly but the outer commit fails and the account.update (stripeCustomerId)
  silently rolls back too. Currently armed only behind the dormant welcome-
  credit flag, but would have broken billing enablement the moment the flag
  flipped. createMany skipDuplicates yields a single ON CONFLICT DO NOTHING
  statement that returns count: 0 cleanly without aborting the TX.
- Apply the same createMany skipDuplicates pattern to the signup-credit route
  too — drops the exception-as-control-flow Prisma namespace import and is
  more uniform with the welcome path.
- Drop the now-orphaned credit_grants_accountId_idx in the same migration.
  The schema removed @@index([accountId]) when @@unique([accountId, reason])
  was added (covered by the leftmost prefix), but the migration only added
  the unique index, leaving prod drifted.

* billing: fix stale finalizeCheckoutSession JSDoc

The function-level JSDoc still described the abandoned try/catch P2002
mechanism after switching to createMany skipDuplicates. The inline
comment + code now agree on the new ON CONFLICT DO NOTHING shape.

* billing: decouple first-card alert, drop vestigial billing field, fix modal cents; sync copy

* docs+homepage: align Router credit copy with signup claim (no card-on-add carrot)

* homepage: add pricing screenshot and pay-as-you-go promo line

* billing: fix once-per-lifetime misframe on first-card alert

* billing: suppress signup credit for prior welcome-credit recipients

* billing: drop bogus '1000 users' cap; invalidate billing on signup-credit settle

main.ts

@@ -183,8 +183,9 @@ function billingConsoleUrl(owner: string, anchor: "billing" | "model-access"): s
  *
  * Branches:
  *   - `router_requires_card`: user is on Router mode with no card AND no
- *     wallet balance. Lead with the carrot ($20 free credit), link to
- *     `#model-access` where the Add Card flow lives.
+ *     wallet balance (signup credit exhausted or not granted). Frame as
+ *     "add a card to continue", link to `#model-access` where the Add
+ *     Card flow lives.
  *   - `router_balance_exhausted`: user has a card on file but auto-reload is
  *     disabled and they've spent past their $5 overdraft buffer. Frame as
  *     "balance ran out" and surface both remediation paths (top up, or flip
@@ -206,7 +207,7 @@ function formatBillingErrorSummary(error: BillingError, owner: string): string {
     return [
       "**Add a card to start using Pullfrog Router.**",
       "",
-      "Router proxies OpenRouter at raw cost — no platform markup, and your first $20 of usage is on us.",
+      "Router proxies OpenRouter at raw cost — no platform markup. Add a card and we'll auto-reload your wallet so runs keep flowing.",
       "",
       `[Add a card →](${billingConsoleUrl(owner, "model-access")})`,
     ].join("\n");