Add plugin purchase refund system

[simonhamp]

Summary

• Adds a complete refund flow for plugin purchases from the Filament admin panel
• Issues a full Stripe refund, revokes the license, and cancels/reverses the developer payout — all from a single button
• Enforces a 14-day refund window; blocks refunds on comped, zero-price, or already-refunded licenses
• Bundle purchases are refunded all-or-nothing (all sibling licenses sharing the same payment intent)
• Refunded licenses are excluded from isActive(), the active() scope, and the sales_view

Changes

• PayoutStatus enum: Added Cancelled case
• PluginPayout model: Added markAsCancelled(), isCancelled(), cancelled() scope
• PluginLicense model: Added isRefunded(), isRefundable(), refundedBy() relationship; updated isActive() and active() scope
• StripeConnectService: Added refundPaymentIntent() and reverseTransfer() methods
• RefundPluginPurchase action: Orchestrates Stripe refund → DB transaction (license revocation + payout cancellation/reversal)
• Filament relation managers: Added "Refund" button (danger, with confirmation modal) and "Refunded" column to both Plugin and User license tables
• Migration: Adds refunded_at, stripe_refund_id, refunded_by to plugin_licenses; updates sales_view to exclude refunded rows
• Factories: New PluginPayoutFactory with transferred()/cancelled() states; added refunded() state to PluginLicenseFactory

Test plan

• 11 feature tests covering all paths (tests/Feature/PluginPurchaseRefundTest.php)
• Manual: Navigate to Plugin → Licenses tab, verify Refund button appears for eligible licenses
• Manual: Navigate to User → Plugins tab, verify Refund button appears and is hidden for ineligible licenses
• Manual: Confirm refund modal shows correct amount, user email, and bundle warning when applicable
• Run full test suite to verify no regressions

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