Phase 10: Per-User Rate Limits — messages + tokens, quota UI, admin usage

Backend:
- max_ai_messages_per_day + max_ai_tokens_per_day on User model (nullable, override)
- Migration 008: add columns + seed default settings (100 msgs, 500K tokens)
- usage_service: count today's messages + tokens, check quota, get limits
- GET /chats/quota returns usage vs limits + reset time
- POST /chats/{id}/messages checks quota before streaming (429 if exceeded)
- Admin user schemas expose both limit fields
- GET /admin/usage returns per-user daily message + token counts
- admin_user_service allows updating both limit fields

Frontend:
- Chat header shows "X/Y messages · XK/YK tokens" with red highlight at limit
- Quota refreshes every 30s via TanStack Query
- Admin usage page with table: user, messages today, tokens today
- Route + sidebar entry for admin usage
- English + Russian translations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/admin/usage.tsx

@@ -0,0 +1,71 @@
+import { Navigate } from "react-router-dom";
+import { useTranslation } from "react-i18next";
+import { useQuery } from "@tanstack/react-query";
+import { useAuthStore } from "@/stores/auth-store";
+import api from "@/api/client";
+import { cn } from "@/lib/utils";
+
+interface UserUsage {
+  user_id: string;
+  username: string;
+  messages_today: number;
+  message_limit: number;
+  tokens_today: number;
+  token_limit: number;
+}
+
+export function AdminUsagePage() {
+  const { t } = useTranslation();
+  const user = useAuthStore((s) => s.user);
+
+  const { data } = useQuery({
+    queryKey: ["admin-usage"],
+    queryFn: async () => {
+      const { data } = await api.get<{ users: UserUsage[] }>("/admin/usage");
+      return data.users;
+    },
+    refetchInterval: 30000,
+  });
+
+  if (user?.role !== "admin") return <Navigate to="/" replace />;
+
+  const users = data || [];
+
+  return (
+    <div className="space-y-4">
+      <h1 className="text-2xl font-semibold">{t("admin_usage.title")}</h1>
+
+      <div className="rounded-lg border">
+        <table className="w-full text-sm">
+          <thead>
+            <tr className="border-b bg-muted/50">
+              <th className="px-4 py-3 text-left font-medium">{t("admin_usage.user")}</th>
+              <th className="px-4 py-3 text-right font-medium">{t("admin_usage.messages")}</th>
+              <th className="px-4 py-3 text-right font-medium">{t("admin_usage.tokens")}</th>
+            </tr>
+          </thead>
+          <tbody>
+            {users.map((u) => (
+              <tr key={u.user_id} className="border-b last:border-0">
+                <td className="px-4 py-3">{u.username}</td>
+                <td className="px-4 py-3 text-right">
+                  <span className={cn(u.messages_today >= u.message_limit && "text-destructive font-medium")}>
+                    {u.messages_today} / {u.message_limit}
+                  </span>
+                </td>
+                <td className="px-4 py-3 text-right">
+                  <span className={cn(u.tokens_today >= u.token_limit && "text-destructive font-medium")}>
+                    {Math.round(u.tokens_today / 1000)}K / {Math.round(u.token_limit / 1000)}K
+                  </span>
+                </td>
+              </tr>
+            ))}
+            {users.length === 0 && (
+              <tr><td colSpan={3} className="px-4 py-8 text-center text-muted-foreground">{t("admin_usage.no_data")}</td></tr>
+            )}
+          </tbody>
+        </table>
+      </div>
+    </div>
+  );
+}