fix: receipt line items collide with invoice UUIDs

Imported from GitHub pull request El-SaMa/oma#125 by @El-SaMa.

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Summary

• Receipts (and credit notes) cloned invoice_items primary keys from the source invoice, so create failed after the receipt header insert; a retry then hit invoices_number_key.
• Allocate fresh line IDs, make receipt creation idempotent (heal empty orphan receipts), and never fall back to stale billing_settings.invoice.nextNumber.
• Drop the n8n checkout temp-URL hint box.

Test plan

• €0 n8n checkout with TEST-ACCOUNT completes without invoice errors
• Paid invoice gets a receipt with its own line item rows
• Re-running receipt creation for the same invoice is a no-op / heal