payments/migration1: add external migration test

Add a developer-facing migration_external_test that allows
running the KV→SQL payments migration against a real channel.db
backend to debug migration failures on actual data. The accompanying
testdata README documents how to supply a database file and configure
the test, so users can validate their data and confirm the migration
completes successfully.

The test is skipped by default and meant for manual diagnostics.

payments/db/migration1/testdata/README.md

@@ -0,0 +1,52 @@
+# Payment Migration External Testdata
+
+This directory holds a real `channel.db` (bbolt) or `channel.sqlite` file for
+testing the payments KV to SQL migration locally. You can also point the test
+at an existing Postgres-backed kvdb instance.
+
+## How to use
+
+1. Copy your `channel.db` or `channel.sqlite` file into this folder.
+2. Edit `migration_external_test.go`:
+
+   ```go
+   // Comment out this line to enable the test
+   t.Skipf("skipping test meant for local debugging only")
+
+   // Set to your database filename
+   const fileName = "channel.db" // or "channel.sqlite"
+   ```
+
+3. Run the test:
+
+   ```bash
+   # For Postgres backend
+   go test -v -tags="test_db_postgres" -run TestMigrationWithExternalDB
+   ```
+
+## SQLite kvdb source
+
+To migrate from a `channel.sqlite` file, run with the `kvdb_sqlite` build
+tag:
+
+```bash
+go test -v -tags="test_db_sqlite kvdb_sqlite" \
+  -run TestMigrationWithExternalDB
+```
+
+## Postgres kvdb source
+
+To migrate from an existing Postgres-backed kvdb instance, edit
+`postgresKVDSN` in `migration_external_test.go` (set it non-empty), then
+run with the `kvdb_postgres` build tag:
+
+```bash
+go test -v -tags="kvdb_postgres test_db_postgres" \
+  -run TestMigrationWithExternalDB
+```
+
+## Notes
+
+- The external database is opened read-only.
+- The test creates a fresh SQL database for each run.
+- Do not commit production data; keep the file local.