Max subscribers: full developer payout + no third-party discounts

[simonhamp]

Summary

• Full payout for Max subscribers: When a Max subscriber purchases a third-party plugin, the developer receives 100% of the sale (0% platform fee instead of the usual 30%)
• No subscriber discounts on third-party plugins: Third-party plugin prices are always shown at the regular price tier — subscriber/EAP discounts only apply to official plugins
• Adds User::hasMaxTierAccess() to detect Max tier via Stripe subscriptions or Anystack licenses
• Adds optional platformFeePercent parameter to PluginPayout::calculateSplit()

Test plan

• Unit tests for calculateSplit() with default, zero, and custom platform fee percentages
• Feature tests for hasMaxTierAccess() across Max, Pro, Mini, and no subscription
• Feature tests verifying third-party plugins always return regular price for all user tiers
• Feature tests verifying official plugins still get subscriber/EAP discounts
• Feature tests for payout creation: Max subscriber → 0% fee on third-party, normal 30% on official
• Feature tests for payout creation: Pro/non-subscriber → normal 30% fee on third-party
• Updated existing tier pricing tests to use official plugins where discounts are expected
• All 67 tests pass

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