7.8 - Fix demo seed scripts: resolve pnpm module layout, read network from env

[Otfrugger]

closes #161

Summary

pnpm demo:seed failed outright, and even fixed it could only ever seed testnet.

1. Module resolution

scripts/demo-seed.ts / demo-reset.ts lived at the repo root and import @stellar/stellar-sdk, which pnpm's strict node_modules layout doesn't let a root-level script resolve. Moved both — plus their shared lib/env.ts and lib/sep10-login.ts helpers — under apps/api/scripts/, following the precedent already set by apps/api/scripts/gen-mainnet-secrets.mjs (whose own header comment explains exactly this constraint). Updated the root demo:seed/demo:reset package.json scripts to run them via pnpm --filter @checkout/api exec tsx scripts/..., matching how secrets:mainnet already invokes gen-mainnet-secrets.mjs.

2. Network hardcoding

demo-seed.ts hardcoded Networks.TESTNET and Friendbot, and refused to run unless GET /health reported "testnet". It now:

• Reads STELLAR_NETWORK (default testnet), HORIZON_URL, and USDC_ISSUER_PUBLIC/USDC_ISSUER_TESTNET the same way apps/api/src/env.ts does — via @checkout/stellar's resolveStellarConfig, so the defaults can never drift out of sync with what the API itself uses.
• Cross-checks its own configured network against the live API's GET /health response and fails with a clear message on mismatch (was previously a hardcoded "testnet" comparison).
• Skips the Friendbot XLM funding step and the testanchor USDC-dispenser step on any non-testnet network, each with an explicit [skip] message, since neither exists off testnet. It doesn't invent an alternative funding mechanism (out of scope) — if the buyer keypair isn't funded some other way, the payment step fails with the real Horizon error (e.g. account-not-found) rather than an obscure one, which is what the issue's acceptance criterion ("works or refuses with a clear reason") asks for.

3. Demo link id

The /demo storefront page's widget button has always linked to data-quay-link="demo_mug_123" (apps/web/app/demo/page.tsx) — but link ids are always a random lnk_... string server-side, so that id could never resolve to a real link (confirmed via git blame, this has been broken since the page's introduction). createLinkSchema (packages/core/src/schemas.ts) now accepts an optional custom id, but rejects it unless isDemo: true is also set, so a real payment link can never be given a predictable id. demo-seed.ts seeds (idempotently — checks GET /links/demo_mug_123 first) a $25.00 link with that exact id, left unpaid so a real visitor can pay it through the widget.

Also updated

• README.md, docs/MAINNET.md, TODO.md — removed/updated the now-stale "testnet-only" caveats.
• apps/api/test/routes/links.test.ts — added coverage for the new id/isDemo schema behavior (custom id accepted with isDemo:true, rejected without it, rejected on bad characters).

Verification

• Read through the full script logic and the resolveStellarConfig/env.ts defaults it now reuses to confirm they match.
• Byte-scanned every changed file for stray control bytes before committing.
• Could not run: no pnpm/node_modules install is available in this environment, so I could not run pnpm typecheck, pnpm test, or actually execute pnpm demo:seed against a live API. Please run those (and a real pnpm demo:seed against a local testnet API) before merging — this is unverified beyond careful manual review.

Caveats

• apps/api/scripts isn't included in apps/api/tsconfig.json's typecheck (include: ["src", "test"]), same as before the move — these scripts were never typechecked by CI either as root-level files or now, so this isn't a regression, but flagging it in case that's worth fixing separately.
• The custom-id schema change is a small, deliberately-scoped API surface addition beyond what the issue's "Key files" list mentioned (packages/core/src/schemas.ts, apps/api/src/services/link-service.ts) — needed to actually seed demo_mug_123. Happy to split it out if preferred.

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