feat: support Backlog Enterprise on-premise custom domains

[VincentZhao]

Description

Summary

bee currently validates the host field in the RC config against a hardcoded regex that only accepts *.backlog.com and *.backlog.jp. Backlog Enterprise (self-hosted) instances use arbitrary custom domains and may include a path prefix (e.g. backlog.example.internal/backlog).

Steps to reproduce

1. Set BACKLOG_SPACE=backlog.example.internal/backlog
2. Run bee auth login → succeeds (login writes the RC directly, bypassing validation)
3. Run bee issue list → fails with a schema validation error

Root cause

packages/config/src/schema.ts line 19:

host: v.pipe(v.string(), v.regex(/^[a-z0-9-]+\.backlog\.(com|jp)$/)),

This regex rejects any host that does not match the SaaS domain pattern.

Expected behavior

All commands work with enterprise on-premise instances that use custom domains.