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/public/locales/en/translation.json

@@ -39,7 +39,8 @@
     "skills": "Skills",
     "personal_context": "My Context",
     "pdf": "PDF Reports",
-    "pdf_templates": "Templates"
+    "pdf_templates": "Templates",
+    "usage": "Usage"
   },
   "dashboard": {
     "welcome": "Welcome, {{name}}",
@@ -56,7 +57,9 @@
     "unarchive": "Unarchive",
     "delete_confirm": "Are you sure you want to delete this chat?",
     "limit_reached": "Chat limit reached",
-    "streaming": "AI is thinking..."
+    "streaming": "AI is thinking...",
+    "quota_messages": "{{used}}/{{limit}} messages",
+    "quota_tokens": "{{used}}K/{{limit}}K tokens"
   },
   "admin": {
     "context_editor": "Primary Context Editor",
@@ -197,6 +200,13 @@
     "default_max_chats": "Default Max Chats",
     "default_max_chats_desc": "Default chat limit for new users"
   },
+  "admin_usage": {
+    "title": "Usage Statistics",
+    "user": "User",
+    "messages": "Messages Today",
+    "tokens": "Tokens Today",
+    "no_data": "No usage data available."
+  },
   "common": {
     "loading": "Loading...",
     "error": "An error occurred",

frontend/public/locales/ru/translation.json

@@ -39,7 +39,8 @@
     "skills": "Навыки",
     "personal_context": "Мой контекст",
     "pdf": "PDF отчёты",
-    "pdf_templates": "Шаблоны"
+    "pdf_templates": "Шаблоны",
+    "usage": "Использование"
   },
   "dashboard": {
     "welcome": "Добро пожаловать, {{name}}",
@@ -56,7 +57,9 @@
     "unarchive": "Разархивировать",
     "delete_confirm": "Вы уверены, что хотите удалить этот чат?",
     "limit_reached": "Достигнут лимит чатов",
-    "streaming": "ИИ думает..."
+    "streaming": "ИИ думает...",
+    "quota_messages": "{{used}}/{{limit}} сообщений",
+    "quota_tokens": "{{used}}K/{{limit}}K токенов"
   },
   "admin": {
     "context_editor": "Редактор основного контекста",
@@ -197,6 +200,13 @@
     "default_max_chats": "Лимит чатов по умолчанию",
     "default_max_chats_desc": "Лимит чатов для новых пользователей"
   },
+  "admin_usage": {
+    "title": "Статистика использования",
+    "user": "Пользователь",
+    "messages": "Сообщения сегодня",
+    "tokens": "Токены сегодня",
+    "no_data": "Данных пока нет."
+  },
   "common": {
     "loading": "Загрузка...",
     "error": "Произошла ошибка",