💳 feat(stripe): add SEPA Direct Debit payment option

- Add sepa_debit to payment_method_types for credit purchases
- Add sepa_debit to subscription checkout sessions
- Handle payment_intent.processing webhook for SEPA status tracking
- Add blueprint article analyzing payment options (Stripe vs LSV+/FinTS)

SEPA offers lower fees (0.8% vs 1.5%+€0.25) for DACH customers.
Payments are confirmed 3-14 days after checkout (bank processing).

services/mana-core-auth/src/subscriptions/subscriptions.service.ts

@@ -139,7 +139,7 @@ export class SubscriptionsService {
 		const session = await this.stripe.checkout.sessions.create({
 			customer: stripeCustomerId,
 			mode: 'subscription',
-			payment_method_types: ['card'],
+			payment_method_types: ['card', 'sepa_debit'],
 			line_items: [
 				{
 					price: stripePriceId,