Global Database 0.1.11 - Cut over legacy mail storage to the global database

Implement the production transition from legacy per-account mail storage to the global mail database. Import durable mail data, validate the result, and establish the durable cutover state that makes global storage authoritative.

Acceptance criteria

• Import covers every mapping in the accepted legacy inventory.
• Derived data, such as search indexes, is rebuilt before validation runs.
• Validation covers imported data, relationships, queued operations, attachments, restart integrity, and search.
• An attachment that fails to copy or validate is recorded in the migration result and fails the migration.
• Before cutover, normal mail access cannot observe imported global data.
• Failed migration leaves legacy storage authoritative.
• Cutover removes legacy database and attachment artifacts.
• After cutover, all mail access uses global storage and cleanup failure does not restore legacy authority.
• If the migration fails due to insufficient storage, the system notifies the user to disable auto-updates and postpone manual version upgrades until sufficient space is cleared.

No files or tests are named. Start by locating the legacy per-account and global mail-storage migration entry points and the accepted legacy inventory, then trace import, derived-data rebuilding, validation, cutover, rollback authority, cleanup, and storage-error handling. Done means every acceptance criterion is covered by the production migration and its validation.