Remove legacy image_data fallback after migration rollout

[catherineluse]

Background

The browser image migration intentionally retains the legacy SQLite image_data fallback so older and partially migrated libraries remain recoverable. Keeping it forever adds complexity and can retain duplicate data.

Scope

• Define the minimum release/telemetry or support window before removal.
• Provide a migration-status check that proves every legacy image has a verified blob copy.
• Offer a recovery/export path for partial migrations.
• Remove legacy read fallback and clear legacy image bytes only after verification.
• Update backup/import compatibility documentation.

Acceptance criteria

• No legacy bytes are deleted when any image migration is partial or unverifiable.
• Fully migrated libraries no longer depend on image_data.
• Backups from supported older versions remain importable.
• Tests cover complete migration, partial migration, retry, and cleanup.