[codex] test creator purchases, posts, wallet auth, and balance sync

[Obiajulu-gif]

Summary

• add XLM balance guarding for creator-key purchases and prevent submission when underfunded
• add Prisma-backed creator posts with authenticated creation and public listing
• add signed Stellar wallet challenge XDRs with unique nonce memos and web_auth_domain
• add an injectable key-balance reconciliation worker with correction logging
• cover all four paths with focused integration and unit tests

Why

The requested test targets were not present on main, so this PR includes the minimal production routes, persistence model, and worker required for the tests to exercise real behavior.

Validation

• pnpm exec prisma generate
• pnpm exec jest --runInBand --runTestsByPath src/jobs/key-balance-sync.worker.test.ts src/modules/creator/buy.integration.test.ts src/modules/creator/post.integration.test.ts src/modules/auth/stellar-challenge.integration.test.ts — 12 tests passed
• pnpm build
• pnpm lint

Closes #703
Closes #712
Closes #717
Closes #718

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