[Part 3b|*] Implement Third Part for SQL Backend functions

[ziggie1984]

Implements the Third and Final Part of Payments SQL backend functions. Follows after
#10291

[ziggie1984]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements the third and final part of the payments SQL backend functions, focusing on payment control and write operations. The changes are extensive and well-executed, including the implementation of functions for initializing, registering, settling, failing, and deleting payments. The database schema has been improved to better model the relationship between payments and payment intents, and performance is enhanced through efficient batching and lightweight queries.

I have two minor suggestions: one to fix a broken link and improve wording in the release notes, and another to address a style guide violation regarding spacing in a switch statement. Overall, this is a solid contribution to the SQL backend migration.

[gemini-code-assist]

The release note entry has a broken link due to a newline, and the wording could be more descriptive and consistent with other entries in this section.

Suggested change

	* Implement third(final) Part of SQL backend [payment functions]
	(https://github.com/lightningnetwork/lnd/pull/10368)
	* Implement payment control functions for the [payments db SQL Backend](https://github.com/lightningnetwork/lnd/pull/10368)

[gemini-code-assist]

The switch statement in the InitPayment function is missing blank lines between case blocks, which is required by the repository style guide for code spacing. Adding blank lines and descriptive comments for each case will improve readability and adhere to the style guide.¹

switch {
		// A payment with this hash already exists. We need to check its
		// status to see if we can re-initialize.
		case err == nil:
			paymentStatus, err := s.computePaymentStatusFromDB(
				ctx, db, existingPayment,
			)
			if err != nil {
				return fmt.Errorf("failed to compute payment "+
					"status: %w", err)
			}

			// Check if the payment is initializable otherwise
			// we'll return early.
			if err := paymentStatus.initializable(); err != nil {
				return fmt.Errorf("payment is not "+
					"initializable: %w", err)
			}

			// If the initializable check above passes, then the
			// existing payment has failed. So we delete it and
			// all of its previous artifacts. We rely on
			// cascading deletes to clean up the rest.
			err = db.DeletePayment(ctx, existingPayment.Payment.ID)
			if err != nil {
				return fmt.Errorf("failed to delete "+
					"payment: %w", err)
			}

		// An unexpected error occurred while fetching the payment.
		case !errors.Is(err, sql.ErrNoRows):
			return fmt.Errorf("failed to check existing "+
				"payment: %w", err)

		// The payment does not yet exist, so we can proceed.
		default:
		}

Style Guide References

Footnotes

1. The style guide requires spacing between case stanzas in a switch statement for better readability. ↩