Fix DB startup crash and explicit shared DB migration

[FilipJaskovic]

This PR fixes DB startup issues around corrupt rows and shared DB migration.

What changed

• Guard DB callback decoding against null/malformed values and skip invalid rows instead of crashing.
• Keep strict fail-fast when shared_database_path (CLI or config) is explicitly set and DB open fails.
• Restore first-run migration path: if explicit shared DB is set, split DB files are missing, and legacy test.db exists, bootstrap from legacy DB once so data is migrated.
• Propagate migration failure to startup by returning bool from ensure_database_compatibility(...) and split_database(...).

[ahrm]

Thanks, since this is a relatively big change I want to implement it myself (I am a little busy right now) but I will definitely use these commits to make sure I have not missed anything, I definitely appreciate your work.