docs: add Buttondown-style add-on pricing model

Commit notes

Added Model B: Base + Add-ons approach inspired by Buttondown: - €4/mo base plan with core features - Modular add-ons: AI Suite, Pro Analysis, Sharing+, Creator Tools, Team - Storage scales at €2/mo per 50GB - Team seats at €3/mo per seat - Bundle discounts for common combinations

Also added: - Model comparison table (simplicity vs flexibility vs revenue) - Stage-based recommendations (early/growing/mature) - Database schema for add-on tracking - Service layer helpers for feature gating

Pragmatic recommendation: Start with four tiers (Model A), evolve to add-ons (Model B) when usage data is available.

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