Add Cloudflare Neon deployment plan

[hudsonaikins]

Summary

• add Cloudflare and Neon deployment plan to source docs
• add Fumadocs deployment page under Operations
• record deploy units, database decision, cost controls, and execution checklist

Validation

• python3 tools/release_check.py
• cd worker && bun run check && bun test

Notes

• No Cloudflare, Neon, DNS, or paid infrastructure was created.
• Plan recommends Neon Postgres first because the CLI and Worker already use Postgres; D1 would require a schema and adapter migration sprint.