feat(db): boot older builds on a newer schema when migrations are backward-compatible

[IbbyLabs]

AIOStreams migrations are append-only, so once a database is migrated by a newer build, an older build refuses to start (Database is at migration N but this build only knows up to M), even when the newer migrations only added things the older code never touches. Rolling back, or trying a newer build and then going back, means manual SQL or wiping the database.

A migration can now declare backwardCompatible when it only adds tables/columns/indexes older code ignores. The runner records that per row in _migrations, and on boot an older build starts against a newer database only if every newer migration is marked compatible. A rename, drop, or retype of an existing table still blocks the boot.

So rolling back a bad upgrade, or trying a build without committing to it, just works for additive changes and stays protected against the ones that would actually break. Nothing to configure.

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[IbbyLabs]

@Viren070 reworked it, migrations now declare if they're backwards compatible and the runner enforces it, breaking ones still block.