Global Database 0.1.4 - Add core database module

Add the database module under core:database with the foundational database implementation.

Take the spike to extract a base database implementation with the Room3 backend.

Provide deterministic schema composition, one coordinated schema and migration history, transactions, and Android/JVM desktop support. Keep the backend domain-neutral. The internal mail database module owns mail schema and storage code. This Feature does not implement mail migration or cutover.

Acceptance criteria

• core:database opens Room 3 on Android and JVM desktop through the documented bootstrap path.
• Schema composition, migrations, and transactions have focused automated coverage.
• Invalid configurations fail with actionable diagnostics.
• core:database public APIs remain library- and domain-neutral.
• Tests demonstrate restart-safe opening and migration behavior on every supported platform.