Human-targeted architecture doc: high-level diagram, 7 Tier-1 + 2 Tier-2 domains, cross-cutting layer, stack-symmetry principle, 6 ADR summaries, layering rule, permission system, and two data-flow walkthroughs (document upload, transcription block autosave). Closes #396 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>