docs: add comprehensive subscription model analysis

Commit notes

Analysis includes: - Current state review identifying lack of value differentiation - Four user personas (Hobbyist, Producer, Creator, Team) - Proposed four-tier subscription model with clear feature gates - Alternative simpler two-tier approach - Messaging and positioning recommendations - Implementation roadmap with database schema changes - Metrics to track for success measurement

Key recommendations: - Reduce free tier to 10GB (from 100GB) to create upgrade incentive - Add Producer tier (€5/mo) for serious library management - Keep Creator tier (€9/mo) for sellers with 0% fees - Add Team tier (€29/mo) for studios/production teams

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