feat: implement Mollie Connect for automated seller payouts

Commit notes

Sellers can now connect their own Mollie accounts to receive payments directly, eliminating manual payouts from the platform.

Changes: - Add Mollie Connect OAuth flow (backend/src/routes/mollie-connect.ts) - Add Mollie Connect service for token management (backend/src/services/mollie-connect.ts) - Add user schema fields for Mollie credentials (mollieOrganizationId, mollieProfileId, etc.) - Update purchase flow to use connected accounts with application fees - Add PLATFORM_FEE_PERCENT config (15% for free tier, 0% for Pro) - Add database migration for new user fields (0015_mollie_connect.sql) - Update env.example with Mollie Connect configuration

How it works: 1. Seller clicks "Connect Mollie Account" -> OAuth flow 2. Seller creates/logs into Mollie, completes KYC 3. Seller's mollieProfileId stored in database 4. When buyer purchases: payment routes to seller's Mollie account 5. Platform takes application fee (15% or 0% based on subscription) 6. Seller receives payment directly from Mollie (no manual payout)

Fallback: If seller hasn't connected, payment goes to platform account and manual payout is required (existing behavior).

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