fix(backup): keep an erased account's copy out of admin accounts

A restore read the account settings under the operator's AI endpoint
policy whenever the receiving account was an admin. For a copy put into
a chosen account, the settings were written by the erased account's
owner, not by the operator, yet an admin account could be chosen as the
target, and an ordinary target promoted later got the same trust.

Admin accounts are now refused as a target, in the check and the
picker, with backup.target.admin_account. A copy uploaded into a chosen
account is stored as RETARGETED_UPLOAD_<ms>, and its restore never reads
its settings as the operator's, whatever the account's role is by then.

docs/api/openapi.yaml

@@ -25189,10 +25189,10 @@ paths:
                 $ref: "#/components/schemas/ErrorEnvelope"
         "409":
           description: "The chosen target cannot take the copy, and nothing was stored: a managed profile
-            (`backup.target.managed_profile`), an account pending deletion (`backup.target.pending_deletion`), an
-            account that already holds records (`backup.target.holds_records`, `meta.models` naming them), a username
-            already taken (`backup.target.username_taken`), or a file whose own account is still here
-            (`backup.owner.exists`)."
+            (`backup.target.managed_profile`), an account pending deletion (`backup.target.pending_deletion`), an admin
+            account (`backup.target.admin_account`), an account that already holds records
+            (`backup.target.holds_records`, `meta.models` naming them), a username already taken
+            (`backup.target.username_taken`), or a file whose own account is still here (`backup.owner.exists`)."
           content:
             application/json:
               schema:
@@ -38717,21 +38717,21 @@ components:
                 auth.session.unavailable, auth.stepup.required, auth.token.expired, auth.token.invalid; backup —
                 backup.foreign_reference, backup.key.missing, backup.owner.exists, backup.owner.missing,
                 backup.payload.undecryptable, backup.restore.active, backup.section.missing,
-                backup.target.holds_records, backup.target.managed_profile, backup.target.not_found,
-                backup.target.pending_deletion, backup.target.unconfirmed, backup.target.username_taken,
-                backup.upload.too_large; biomarkers — biomarkers.unit.locked; body-site — body-site.invalid; coach —
-                coach.budget.exceeded, coach.conversation.invalidTitle, coach.conversation.notFound,
-                coach.fenced.attachmentConflict, coach.fenced.attachmentLimit, coach.fenced.attachmentNotFound,
-                coach.fenced.attachmentNotIndexed, coach.fenced.attachmentRequired, coach.fenced.attachmentUnavailable,
-                coach.fenced.invalid, coach.request.invalid; coach-prefs — coach-prefs.body.invalid_json,
-                coach-prefs.body.invalid_shape; consent — consent.ai.confirmRequired, consent.ai.required; cycle —
-                cycle.bulk.invalid, cycle.bulk.too_large, cycle.calendar.invalid, cycle.calendar.range,
-                cycle.cycles.invalid, cycle.day-log.conflict, cycle.day-log.invalid, cycle.disabled,
-                cycle.period.invalid, cycle.symptom.custom.invalid, cycle.symptom.custom.limit; cycle-prefs —
-                cycle-prefs.body.invalid_json, cycle-prefs.invalid; disable-coach — disable-coach.body.invalid_json;
-                data_wipe — data_wipe.copy_failed; dawarich — dawarich.credentials_rejected,
-                dawarich.home_place_unknown, dawarich.invalid, dawarich.not_connected, dawarich.rate_limited,
-                dawarich.unreachable; day — day.invalid; demo — demo.readOnly; documents —
+                backup.target.admin_account, backup.target.holds_records, backup.target.managed_profile,
+                backup.target.not_found, backup.target.pending_deletion, backup.target.unconfirmed,
+                backup.target.username_taken, backup.upload.too_large; biomarkers — biomarkers.unit.locked; body-site —
+                body-site.invalid; coach — coach.budget.exceeded, coach.conversation.invalidTitle,
+                coach.conversation.notFound, coach.fenced.attachmentConflict, coach.fenced.attachmentLimit,
+                coach.fenced.attachmentNotFound, coach.fenced.attachmentNotIndexed, coach.fenced.attachmentRequired,
+                coach.fenced.attachmentUnavailable, coach.fenced.invalid, coach.request.invalid; coach-prefs —
+                coach-prefs.body.invalid_json, coach-prefs.body.invalid_shape; consent — consent.ai.confirmRequired,
+                consent.ai.required; cycle — cycle.bulk.invalid, cycle.bulk.too_large, cycle.calendar.invalid,
+                cycle.calendar.range, cycle.cycles.invalid, cycle.day-log.conflict, cycle.day-log.invalid,
+                cycle.disabled, cycle.period.invalid, cycle.symptom.custom.invalid, cycle.symptom.custom.limit;
+                cycle-prefs — cycle-prefs.body.invalid_json, cycle-prefs.invalid; disable-coach —
+                disable-coach.body.invalid_json; data_wipe — data_wipe.copy_failed; dawarich —
+                dawarich.credentials_rejected, dawarich.home_place_unknown, dawarich.invalid, dawarich.not_connected,
+                dawarich.rate_limited, dawarich.unreachable; day — day.invalid; demo — demo.readOnly; documents —
                 documents.chat.conversationNotFound, documents.chat.invalid, documents.inbound.alreadyConfirmed,
                 documents.inbound.alreadyPartlyConfirmed, documents.inbound.budgetExceeded,
                 documents.inbound.decryptFailed, documents.inbound.encounterNotFound, documents.inbound.episodeNotFound,