feat(settings): serve admin user-settings through the canonical API

Replace the ten string-registry /admin/users/{id}/settings* and
device-settings* routes with the canonical contract surface: one list of
every explicit value the target user has stored across all scopes, and
set/delete at an explicit scope named in the query string.

The admin handlers live on SettingValuesHandler and share the session
routes' implementation rather than duplicating it — the same key/scope
parsing, identity validation, contract scope allowance, value
normalization and mutation-receipt idempotency, factored into
keyedScopeFromRequest/completeIdentity and setValueAt/deleteValueAt.
The only admin-specific parts are the target user coming from the path,
profile and device ids coming from the query (an admin holds no session
claim to the user being inspected, so its named profile is checked to
exist), and change events attributed to the target user so their
clients refresh.

The list is a new UserStore read, ListAllSettingValues, implemented in
both backends and pinned by the shared storetest conformance suite:
the admin surface wants the stored truth (which overrides exist, for a
per-row reset affordance), which no resolution-shaped read answers.

The ten removed routes are recorded in the pre-lock removals table in
docs/architecture/v1-scope.md per the v1 API rules; the web admin
device-overrides page moves onto the new surface in the Phase B
rewrite inside this same unmerged PR.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/architecture/v1-scope.md

@@ -32,6 +32,7 @@ justification and falls back to the Deprecation/Sunset flow like anything else.
 | Removed | Release | Rationale |
 |---|---|---|
 | String `GET`/`PUT`/`DELETE /api/v1/settings…`, the unknown-key extension bag, preference fields on profile/library/series DTOs | Cross-platform settings contract, [design](../superpowers/specs/2026-07-10-cross-platform-user-settings-contract-design.md) | Replaced wholesale by the typed settings contract. Deferring past lock would mean carrying the Deprecation/Sunset surface *and* the untyped key bag — which lets any client invent a production setting the server stores unvalidated — through the deprecation window, which is the exact surface the contract exists to close. |
+| The ten string-registry admin user-settings routes: `GET /api/v1/admin/users/{id}/settings`, `GET /api/v1/admin/users/{id}/settings/{key}`, `PUT /api/v1/admin/users/{id}/settings/{key}`, `DELETE /api/v1/admin/users/{id}/settings/{key}`, `GET /api/v1/admin/users/{id}/device-settings`, `GET /api/v1/admin/users/{id}/device-settings/{key}`, `DELETE /api/v1/admin/users/{id}/device-settings/{key}`, `PUT /api/v1/admin/users/{id}/profiles/{profile_id}/device-settings/{key}/{device_id}`, `DELETE /api/v1/admin/users/{id}/profiles/{profile_id}/device-settings/{key}/{device_id}`, `DELETE /api/v1/admin/users/{id}/profiles/{profile_id}/devices/{device_id}/settings` | Cross-platform settings contract, [design](../superpowers/specs/2026-07-10-cross-platform-user-settings-contract-design.md) | The admin projection of the removal above: these routes read and wrote the string registry the contract replaces. Their canonical successors are `GET /api/v1/admin/users/{id}/settings/values` (every stored value across all scopes) and `PUT`/`DELETE /api/v1/admin/users/{id}/settings/values/{key}` at an explicit scope, sharing the session routes' validation. Keeping the string routes past lock would preserve an admin-only write path into the untyped bag after the user-facing one closed. |
 
 Feature-detection precedent: clients discover which metadata providers (including the
 built-in NFO provider, #216) apply to a library type via