Adds BGType persistence (0=Deck/1=CustomBG/2=RandomBG) to Viewer via two scalar
columns and an owned collection keyed (ViewerId, Slot). Two persistence tests
confirm round-trip and zero-defaults on fresh viewers.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Backstop for /tool/signup idempotency: signup-created viewers carry
the client's UDID (the AES key for that client's wire traffic);
admin-imported viewers stay null. Partial unique index allows the
column to coexist with pre-existing null rows.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
