sqldb+payments: add payment_duplicates for legacy duplicate payments

Older LND versions could create multiple payments for the same hash.
We need to preserve those historical records during KV→SQL migration,
but they don’t fit the normal payment schema because we enforce a
unique payment hash constraint. Introduce a lean payment_duplicates
table to store only the essential fields (identifier, amount,
timestamps, settle/fail data).

This keeps the primary payment records stable and makes the migration
deterministic even when duplicate records lack attempt info. The table
is intentionally minimal and can be dropped after migration if no
duplicate payments exist.

For now there is no logic in place which allows the noderunner to
fetch duplicate payments after the migration.

sqldb/sqlc/querier.go

@@ -49,6 +49,9 @@ type Querier interface {
 	FetchHtlcAttemptResolutionsForPayments(ctx context.Context, paymentIds []int64) ([]FetchHtlcAttemptResolutionsForPaymentsRow, error)
 	FetchHtlcAttemptsForPayments(ctx context.Context, paymentIds []int64) ([]FetchHtlcAttemptsForPaymentsRow, error)
 	FetchPayment(ctx context.Context, paymentIdentifier []byte) (FetchPaymentRow, error)
+	// Fetch all duplicate payment records from the payment_duplicates table for
+	// a given payment ID.
+	FetchPaymentDuplicates(ctx context.Context, paymentID int64) ([]PaymentDuplicate, error)
 	FetchPaymentLevelFirstHopCustomRecords(ctx context.Context, paymentIds []int64) ([]PaymentFirstHopCustomRecord, error)
 	// Batch fetch payment and intent data for a set of payment IDs.
 	// Used to avoid fetching redundant payment data when processing multiple
@@ -177,6 +180,9 @@ type Querier interface {
 	// payment process.
 	InsertPayment(ctx context.Context, arg InsertPaymentParams) (int64, error)
 	InsertPaymentAttemptFirstHopCustomRecord(ctx context.Context, arg InsertPaymentAttemptFirstHopCustomRecordParams) error
+	// Insert a duplicate payment record into the payment_duplicates table and
+	// return its ID.
+	InsertPaymentDuplicateMig(ctx context.Context, arg InsertPaymentDuplicateMigParams) (int64, error)
 	InsertPaymentFirstHopCustomRecord(ctx context.Context, arg InsertPaymentFirstHopCustomRecordParams) error
 	InsertPaymentHopCustomRecord(ctx context.Context, arg InsertPaymentHopCustomRecordParams) error
 	// Insert a payment intent for a given payment and return its ID.