Fix Pro-tier margin leak and pricing-model integrity

Commit notes

Implements the decided hybrid pricing model (free 15% / Pro+Team 3%, storage-tiered subscription) and closes the unit-economics gap it had.

- Add a fixed per-transaction component (PLATFORM_FEE_FIXED_CENTS, default €0.35) on top of the percentage platform fee. Stripe Connect destination charges deduct the processor's fixed cost (~€0.30) from the platform's application fee, so a pure-percentage fee at the reduced 3% Pro rate netted the platform a loss on most sample-pack sales. The fixed component is capped at the sale price (seller earnings never negative) and never applies to free items. - Export calculatePlatformFeeForAmount and add unit tests covering free/Pro splits, the zero-price guard, the negative-earnings cap, and the processor-cost invariant. - Fix stale "0% for Pro" comments in purchases.ts (the code charges 3%). - Remove the unimplemented "Priority audio analysis" claim from the landing page (analysis jobs are not enqueued with plan-based priority). - Add PRICING.md as the single source of truth and mark the three superseded pre-Stripe business-model docs as historical.

[private session redacted]