feat(admin-console): Stage 3 — users domain

Add the operator console's user-administration pages over the existing
*user.Service (no new business logic).

- GET  /_gm/users            paginated account list
- GET  /_gm/users/{id}       account detail: profile, entitlement, sanctions
- POST /_gm/users/{id}/block        apply permanent_block (reason required)
- POST /_gm/users/{id}/entitlement  set the entitlement tier
- POST /_gm/users/{id}/soft-delete  soft-delete the account (cascades)

The console depends on a UserAdmin interface (satisfied by *user.Service) so the
pages render in tests without a database. All writes flow through the CSRF
guard, carry the operator as the audit actor, and answer with a 303 redirect;
a generic message page handles not-found, validation, and failure notices.
Unblock is intentionally absent — the admin API exposes no remove-sanction
endpoint.

Tests: list/detail render, not-found, block (with actor/scope/reason
assertions), missing-reason 400, bad-CSRF 403, entitlement, soft-delete
redirect, and the service-unavailable path.

Docs: backend/docs/admin-console.md gains the page inventory.

backend/internal/adminconsole/users.go

@@ -0,0 +1,61 @@
+package adminconsole
+
+// UserRow is one line in the users list table.
+type UserRow struct {
+	UserID      string
+	Email       string
+	UserName    string
+	DisplayName string
+	Tier        string
+	Blocked     bool
+	Deleted     bool
+	CreatedAt   string
+}
+
+// UsersListData is the view model for the paginated users list.
+type UsersListData struct {
+	Items    []UserRow
+	Page     int
+	PageSize int
+	Total    int
+	HasPrev  bool
+	HasNext  bool
+	PrevPage int
+	NextPage int
+}
+
+// SanctionView is one active sanction shown on the user detail page.
+type SanctionView struct {
+	SanctionCode string
+	Scope        string
+	ReasonCode   string
+	AppliedAt    string
+	ExpiresAt    string
+}
+
+// UserDetailData is the view model for a single user's detail page,
+// combining the account aggregate with the form option lists.
+type UserDetailData struct {
+	UserID            string
+	Email             string
+	UserName          string
+	DisplayName       string
+	PreferredLanguage string
+	TimeZone          string
+	DeclaredCountry   string
+	Blocked           bool
+	Deleted           bool
+	CreatedAt         string
+	UpdatedAt         string
+
+	Tier              string
+	IsPaid            bool
+	EntitlementSource string
+	EntitlementReason string
+	EntitlementEnds   string
+
+	Sanctions []SanctionView
+
+	// Tiers lists the selectable entitlement tiers for the form.
+	Tiers []string
+}