docs: add threat intel system docs and update architecture overview

- Add docs/threat-intel.md: detailed threat intel pipeline documentation
  (DB schema, sync infrastructure, check flow, cross-product integration)
- Update docs/DOSafe-Architecture.md: comprehensive system overview
  covering AI detection, scam check, bot, extension, and threat intel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JOY