Database corruption handling

In case the database gets corrupted, most likely during a migration, we pretty much fail at the first request. To put it mildly, we need a better handling.

• Add some sanity checks for all tables & triggers
• Add some migration & sanity checks progress callbacks
• Add in-memory playlist export
• Add playlist restoration
• Rename & repurpose DbReset to DbCorrupted to let the application handle it
• Add a public function to wipe the database & restore the backed up playlists