docs: storage limits, Cloudflare deploy, and storage error reference

[openrijal]

Closes #4.

Summary

Adds three operational docs that cover production gaps in JSON admin:

• docs/storage-limits.md — GitHub Contents API 1 MB inline limit, Blobs API 100 MB ceiling, sizing table by row size, write amplification, and when to move off single-file `array` storage.
• docs/deploying-cloudflare-pages.md — per-request token-injection middleware pattern for runtimes where `runtimeConfig` is frozen and secrets live on `event.context.cloudflare.env` instead of `process.env`. Includes a runnable middleware example, env-var checklist, and verification steps.
• docs/error-reference.md — every `createError` thrown by `server/utils/githubContents.ts` and `server/utils/jsonStorage/*` with status code, exact message, cause, and remediation. Reflects the post-fix: fall back to git blobs API for github files >1MB #3 Blobs API fallback path.

Cross-links the new pages from `README.md` and `docs/json-admin.md`.

Audit notes

A prior external audit claimed `docs/json-admin.md`, `docs/autoadmin-roles.md`, and `docs/development.md` were truncated mid-sentence. I verified each against actual repo source — they are not truncated. The audit also flagged missing reference docs (full `nuxt.config.ts`, auth integration guide, S3 walkthrough, DB migration guide). Those are real but out of scope here; happy to take them on as follow-ups.

Test plan

• Verified every message in the error reference exists in source (grepped each `createError` call against the listed file).
• Verified middleware example compiles against current types in the repo.
• Sizing-table numbers are approximate by design — comfortable/slow/ceiling bands tie back to the documented GitHub limits.
• Render check in your docs site / GitHub markdown viewer.