Add contribution payment options to registries

- Added support for single and amount-specific QR codes.
- Updated `RegistrySettings` model and database schema.
- Enhanced `RegistryAdmin` to configure payment options.
- Introduced `RegistryContributionAmount` for partial fulfillment.
- Updated `RegistryPublic` to display contribution progress.
- Added new models for payment methods and QR code handling.
- Improved privacy by limiting contributor visibility.

.github/copilot-instructions.md

@@ -6,6 +6,7 @@
 - Authentication must support local accounts plus Microsoft and Google login; admin invite links are single-use and not email-locked.
 - Money participation defaults to EUR and tracks fulfilled amount per item via bank-transfer contributions with item-linked messages.
 - Registry visibility is public-by-link initially, but logged-in users who visited a registry should be able to rediscover it in-app.
+- Identities must be hidden from non-admin users for privacy, only admins and the user who purchased/contributed can see that he did so. Other users may see a generic indication that a contrubution/purchase was made but never a name/email of another user.
 - Use SQL Server as the default provider and set `RequireConfirmedAccount=false` for MVP testing; the first registered account should be the owner with full access.
 
 ## Technical Specifications