Apply the virtual scope chain to the entity invitation entity

Settle every scope operation of the `entity_invitation` entity on the virtual scope chain. The wiring declares 4 scope operations for it; each one is accounted for below. Resolve ownership through `entity_memberships` joined to `virtual_scopes`; helpers live in `src/ai/backend/manager/models/virtual_scope/queries.py`.

## Convert

- `search_entity_invitations` — two of its scope items: `EntityInvitationInviteeScope` (`models/entity_invitation/scopes.py:26`), an email subquery, and `EntityInvitationInviterScope` (`:55`), `EntityInvitationRow.inviter_user_id`

## Already settled

- `accept_entity_invitation`, `reject_entity_invitation` — status transitions on a named invitation, not scoped lists

## Out of scope

- `EntityInvitationTargetScope` — the third scope item of the same operation; it reads the offered entity, which is a parent read

## Write side

Missing for both arms. `models/entity_invitation/creators.py:42` declares `member_of() -> (target,)` only, so `create_entity_invitation` records neither the invitee nor the inviter. The write side has to land first.

## Done when

- The invitee and inviter edges are written at creation and backfilled for existing rows.
- Neither scope references the invitee email subquery nor the inviter column.
- Each converted read has a test proving it returns the same rows as the column-based query on backfilled data.

JIRA Issue: BA-7550

Start with models/entity_invitation/creators.py:42 and the helpers in src/ai/backend/manager/models/virtual_scope/queries.py, then inspect the invitee and inviter scope definitions in models/entity_invitation/scopes.py. Implement and test the write-side edges and backfill before converting search_entity_invitations. Done means both reads match the column-based query on backfilled data and no longer reference the email subquery or inviter column.

Written by the indexing model from the issue text.