* fix: allow private URLs by default for reverse proxy setups
Fixes#588 - Users with reverse proxy setups (e.g., Antigravity tools)
were getting "Invalid base URL" errors due to SSRF protection blocking
private/internal URLs.
Changes:
- Add ALLOW_PRIVATE_URLS env var (defaults to true)
- Set to "false" to enable strict SSRF protection if needed
* refactor: extract isPrivateUrl to shared utility
