Global Database 0.1.5 - Add mail database module

Implement the internal mail database module that provides one global, legacy-compatible mail store for the application. It is the persistence implementation used by the global repository implementations.

The module owns mail schema, mappings, local data sources, and attachment access. core:database owns the domain-neutral Room 3 backend. Migration, validation, and cutover are separate work.

Acceptance criteria

• The module provides one global mail store for the application.
• It represents every durable mail record, queued operation, and downloaded attachment in the legacy table inventory.
• Account scope remains explicit and global identifiers are available where repository contracts require them.
• Downloaded attachment content remains file-backed.
• Mail schema and storage implementation types remain internal and do not escape repository contracts.
• The implementation supports Android and JVM desktop.
• Tests cover schema creation, migrations, transactions, and restart recovery.