[codex] Sharding governance interview notes

[loulanyue]

Summary

• Add database architecture guidance covering read-write splitting, sharding triggers, shard key selection, cross-shard queries, and zero-downtime migration.
• Keep the content aligned with agent_java_offer's backend interview focus while preserving interview-notes structure.

Target file

• 03-database/mysql/1.数据库架构.md

Validation

• node scripts/run-maintenance-checks.mjs

Closes #102