feat(admin): let an admin delete another account through the grace window

There was no way for an admin to remove someone else's account; the
users list only offered the undo for a deletion the account itself or a
Guardian had asked for.

DELETE /api/admin/users/{id} takes the account's own deletion path,
now shared as requestAccountDeletion: with a grace window the account
is locked at once (credentials revoked, record untouched) and erased by
the purge job when the window ends, and the existing undo gives it back
until then; with ACCOUNT_DELETION_GRACE_DAYS=0 it is erased in the
request. The route is cookie-only behind requireAdmin and the
recent-proof gate, takes the typed DELETE_ACCOUNT confirmation, and
refuses the caller's own account, a managed profile (its Guardians
delete it), an account already pending deletion, the last admin who
can sign in and the last Guardian of a managed profile, the last three
decided under their locks inside the transaction. Both the admin and
the account get an audit row.

The users list says who asked for a pending deletion, now including an
admin, and flags managed profiles. Each other account's row carries a
delete button whose confirmation names the purge date, or says the
erasure is immediate when the server keeps no window.

docs/api/openapi.yaml

@@ -38543,8 +38543,9 @@ components:
                 `upstream_error`, `timeout`, `connection_failed` and the per-provider additions) are enumerated in each
                 `/test` operation instead, where the differences can be stated; and a 401 raised by a route checking a
                 credential of its own carries the generic `http.unauthenticated`. account —
-                account.deletion.no_guardian, account.deletion.not_pending; activity — activity.calendar.future_date,
-                activity.calendar.too_old; ai — ai.budget.exceeded, ai.provider.none, ai.record.notPermitted,
+                account.deletion.already_pending, account.deletion.no_guardian, account.deletion.not_pending; activity —
+                activity.calendar.future_date, activity.calendar.too_old; admin — admin.users.delete.managed_profile,
+                admin.users.delete.self; ai — ai.budget.exceeded, ai.provider.none, ai.record.notPermitted,
                 ai.unavailable, ai.usage.aboveCeiling, ai.usage.defaultAboveCeiling, ai.usage.invalid,
                 ai.usage.levelTooLow, ai.usage.rateLimited; ai_provider — ai_provider.invalid, ai_provider.no_fields;
                 aiRuns — aiRuns.failed, aiRuns.notFound, aiRuns.readerBusy, aiRuns.timedOut, aiRuns.workerUnavailable;