docs: add Zeabur deployment readiness

[yihui-dev]

What changed

• Added the 17old Zeabur readiness note for the OpenAnalytics fork.
• Recorded the version baseline, server purchase floor, DNS prerequisites, and the fact that Zeabur does not directly deploy Docker Compose.

Why

The project requires a multi-service deployment. This avoids treating it as a single Node service before the final Zeabur topology and domain are chosen.

Validation

• pnpm run verify
• pnpm exec tsc -p tests/tsconfig.json
• pnpm run tracker:build
• pnpm run test (2,991 tests)

Deployment impact

No runtime services, DNS records, domains, or secrets were created. The PR is intentionally draft until the server and deployment path are chosen.

[github-actions]

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