docs: update PRD with monorepo architecture (v4)

Major updates to Technical Architecture section:
- Document monorepo structure (apps/web, apps/worker, packages/*)
- Add architecture principles (no heavy analysis in Vercel)
- Add detailed system diagram showing data flow
- Document worker job claiming pattern (FOR UPDATE SKIP LOCKED)
- Add GitHub token handling patterns (encrypted storage)
- Clarify analysis phases (Phase 0-1: API only, Phase 2+: cloning)
- Update Next.js route map

Data Model updates:
- Add github_accounts table for token storage
- Update ERD to show new table
- Add RLS policy for github_accounts

Implementation Plan updates:
- Mark completed tasks (project setup, local Supabase)
- Update tasks to reference monorepo locations
- Clarify worker vs web responsibilities

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

Author: devakone