payments/migration1: freeze core payment code

Copy the core payments/db code into payments/db/migration1 and
add the required sqlc-generated types/queries from sqldb/sqlc.
This effectively freezes the migration code so it stays robust
against future query or schema changes in the main payments package.

Replace the delegation to channeldb.ReadElement/WriteElement with
self-contained, frozen implementations that only handle the exact types
required by this migration package. This removes the dependency on the
live channeldb codec so that future changes to channeldb serialization
cannot silently corrupt or break the migration.

UnknownElementType is also defined locally for the same reason.

payments/db/migration1/sqlc/db.go

@@ -0,0 +1,31 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.29.0
+
+package sqlc
+
+import (
+	"context"
+	"database/sql"
+)
+
+type DBTX interface {
+	ExecContext(context.Context, string, ...interface{}) (sql.Result, error)
+	PrepareContext(context.Context, string) (*sql.Stmt, error)
+	QueryContext(context.Context, string, ...interface{}) (*sql.Rows, error)
+	QueryRowContext(context.Context, string, ...interface{}) *sql.Row
+}
+
+func New(db DBTX) *Queries {
+	return &Queries{db: db}
+}
+
+type Queries struct {
+	db DBTX
+}
+
+func (q *Queries) WithTx(tx *sql.Tx) *Queries {
+	return &Queries{
+		db: tx,
+	}
+}