docs: add business model review with critical findings

Commit notes

Key findings: - Storage claim in UI is misleading (says '50x more' but both tiers get 100GB) - Pro features list includes unimplemented items (priority processing) - Adverse selection economics: power sellers pay flat fee, casual sellers pay % - No monetization path for buyers - Marketplace fee structure is sound

Recommendations: - Fix false storage claims (legal risk) - Differentiate storage limits (e.g., 5GB free, 100GB pro) - Consider 5% Pro fee instead of 0% to capture power seller revenue - Add buyer monetization (subscriptions, credits)

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