docs: add revenue potential calculations for each model

Commit notes

Comprehensive financial analysis including: - User distribution assumptions (10k users, 5-8% conversion) - MRR/ARR calculations for all three models - Revenue per user (RPU) and ARPU metrics - Scaling projections to 50k and 100k users - Marketplace commission revenue modeling - Break-even analysis with cost assumptions - Lifetime value (LTV) comparison - Sensitivity analysis for conversion rates

Key findings: - Model B (Add-ons): €8,700 MRR / €104k ARR (+80% vs current) - Model A (Four Tiers): €6,805 MRR / €82k ARR (+41% vs current) - Model C (Two Tiers): €4,831 MRR / €58k ARR (baseline) - At 100k users: Model B generates €1M+ ARR - 80% gross margins achievable

Co-authored-by: armin.naimi <[email redacted]>