docs: clarify setup costs and Link wallet onboarding

[fmhall]

The README omitted the Vercel plan requirement and left Link wallet setup too brief for a new installation. This cleanup gives readers a deploy, phone-verification, and optional-account-connection flow, with storage internals and compatibility notes in collapsible sections.

Adds free-tier guidance for supporting services, explains why the every-minute schedule requires Vercel Pro, and clarifies metered Vercel usage. Link setup now covers Stripe OAuth registration, exact production and local callback URLs, credential sources, environment settings, redeployment, and how to confirm a connected wallet.

Validation:

• pnpm check — passed, including 857 tests across 95 files.
• pnpm build — passed.
• pnpm exec oxfmt --check README.md and git diff --check — passed.
• Reviewed the setup steps against repository configuration and current Stripe/Vercel documentation; no live wallet connection was performed.

Only README.md changes.