get/read ownership transfer

[DNR500]

Summary WIP

Backend for #1767 / #1770.

Built on top of #1019 (nominated_user_id / requested_by_user_id / status schema for ownership_transfers), which lays the data model but doesn't add any endpoint yet. This PR should be rebased onto main once #1019 merges — as the base branch #1019 develops it will mainly be concerned with the endpoint to start a transfer, so this PR is scoped to the get/read endpoint that provides the status of a transfer.

Adds the endpoint editor-standalone needs to show ownership transfer status to both the owner and the nominee.

What's changed?

• GET /api/schools/:school_id/ownership_transfer — returns who the current user is relative to the pending transfer; anyone not involved gets a 404, same as if no transfer existed
• Closes a race condition Create ownership transfer #1019 flagged as not yet covered: a partial unique index on ownership_transfers.school_id (scoped to status = 'pending') plus a matching model validation, so two concurrent requests can't both succeed
• Small fix to a pre-existing bug in Create ownership transfer #1019's own mailer spec (unrelated to the above — the bare factory's random nominated_user_id never had a role anywhere, so create(:ownership_transfer) always failed validation)

Examples

Checking transfer status

GET /api/schools/9d9c1e3e-1e4b-4b0a-9a2e-6a2b7e6f1a10/ownership_transfer
Authorization: Bearer <token>

As the owner who requested it, while it's pending:

// 200 OK
{
  "status": "pending",
  "you_are": "owner",
  "nominee_name": "Priya Shah"
}

As the nominee, while it's pending:

// 200 OK
{
  "status": "pending",
  "you_are": "nominee"
}

The transfer stays visible to the requester and nominee after it's resolved — status reflects whatever actually happened, rather than the response disappearing once it's no longer pending:

// 200 OK — the requester, after the nominee accepted
{
  "status": "completed",
  "you_are": "owner",
  "nominee_name": "Priya Shah"
}

// 200 OK — the nominee, after declining it themselves
{
  "status": "rejected",
  "you_are": "nominee"
}

As anyone else at the school (a different teacher, a different owner who wasn't involved, or if no transfer has ever existed for the school) — deliberately indistinguishable, so the response doesn't leak whether one exists or what happened to it:

404 Not Found

(empty body)

Other responses: 401 (no token), 403 (caller has no role at that school at all).

[github-actions]

Test coverage

93.59% line coverage reported by SimpleCov.
Run: https://github.com/RaspberryPiFoundation/editor-api/actions/runs/34990734477