Merge jay backend into frontend branch

Resolves all 4 conflicts:
- .gitignore: combined Python/Node/Browser Use ignores from both branches
- PRD.md: jay v4 as base + frontend data model, P&L, and implementation status sections
- FRONTEND-BACKEND-CORE-DIFFERENCES.md: kept jay's (grounded in real backend code)
- FRONTEND-BACKEND-RECONCILIATION-PLAN.md: kept jay's (7-phase plan from actual implementation)

Adds next_steps.md as the merge/integration roadmap.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>