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/assets/console.css

@@ -69,3 +69,32 @@ h1 { font-size: 1.4rem; margin: 0 0 0.4rem; }
 .errors ul { margin: 0; padding-left: 1.1rem; color: var(--danger); }
 .ok { color: var(--ok); }
 .bad { color: var(--danger); }
+
+.list { width: 100%; border-collapse: collapse; font-size: 0.9rem; margin-bottom: 1rem; }
+.list th, .list td { text-align: left; padding: 0.35rem 0.6rem; border-bottom: 1px solid var(--line); }
+.list th { color: var(--ink-dim); font-weight: 600; }
+.list tr:hover td { background: var(--panel-hi); }
+.pager { display: flex; gap: 1rem; align-items: center; color: var(--ink-dim); }
+.form { display: flex; flex-wrap: wrap; gap: 0.6rem; align-items: end; margin-top: 0.8rem; }
+.form label { display: flex; flex-direction: column; gap: 0.2rem; font-size: 0.85rem; color: var(--ink-dim); }
+.form input, .form select {
+  background: var(--bg);
+  color: var(--ink);
+  border: 1px solid var(--line);
+  border-radius: 6px;
+  padding: 0.35rem 0.5rem;
+  font: inherit;
+}
+button {
+  background: var(--accent);
+  color: #06121f;
+  border: 0;
+  border-radius: 6px;
+  padding: 0.4rem 0.9rem;
+  font: inherit;
+  font-weight: 600;
+  cursor: pointer;
+}
+button:hover { filter: brightness(1.1); }
+button.danger { background: var(--danger); color: #1a0606; }
+code { background: var(--bg); padding: 0.05rem 0.3rem; border-radius: 4px; }