Phase 2: Chat & AI Core — Claude API streaming, chat UI, admin context

Backend:
- Chat, Message, ContextFile models + Alembic migration
- Chat CRUD with per-user limit enforcement (max_chats)
- SSE streaming endpoint: saves user message, streams Claude response,
  saves assistant message with token usage metadata
- Context assembly: primary context file + conversation history
- Admin context CRUD (GET/PUT with version tracking)
- Anthropic SDK integration with async streaming
- Chat ownership isolation (users can't access each other's chats)

Frontend:
- Chat page with sidebar chat list + main chat window
- Real-time SSE streaming via fetch + ReadableStream
- Message bubbles with Markdown rendering (react-markdown)
- Auto-growing message input (Enter to send, Shift+Enter newline)
- Zustand chat store for streaming state management
- Admin primary context editor with unsaved changes warning
- Updated routing: /chat, /chat/:chatId, /admin/context
- Enabled Chat and Admin sidebar navigation
- English + Russian translations for all new UI

Infrastructure:
- nginx: disabled proxy buffering for SSE support
- Added ANTHROPIC_API_KEY and CLAUDE_MODEL to config

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

frontend/public/locales/en/translation.json

@@ -42,6 +42,28 @@
     "welcome": "Welcome, {{name}}",
     "subtitle": "Your personal AI health assistant"
   },
+  "chat": {
+    "new_chat": "New Chat",
+    "no_chats": "No chats yet. Create one to get started.",
+    "no_messages": "Start a conversation...",
+    "select_chat": "Select a chat to start messaging",
+    "type_message": "Type a message...",
+    "send": "Send",
+    "archive": "Archive",
+    "unarchive": "Unarchive",
+    "delete_confirm": "Are you sure you want to delete this chat?",
+    "limit_reached": "Chat limit reached",
+    "streaming": "AI is thinking..."
+  },
+  "admin": {
+    "context_editor": "Primary Context Editor",
+    "context_placeholder": "Enter the system prompt for the AI assistant...",
+    "save": "Save",
+    "saved": "Saved",
+    "unsaved_changes": "Unsaved changes",
+    "version": "Version",
+    "characters": "characters"
+  },
   "common": {
     "loading": "Loading...",
     "error": "An error occurred",

frontend/public/locales/ru/translation.json

@@ -42,6 +42,28 @@
     "welcome": "Добро пожаловать, {{name}}",
     "subtitle": "Ваш персональный ИИ-ассистент по здоровью"
   },
+  "chat": {
+    "new_chat": "Новый чат",
+    "no_chats": "Пока нет чатов. Создайте первый.",
+    "no_messages": "Начните разговор...",
+    "select_chat": "Выберите чат для начала общения",
+    "type_message": "Введите сообщение...",
+    "send": "Отправить",
+    "archive": "Архивировать",
+    "unarchive": "Разархивировать",
+    "delete_confirm": "Вы уверены, что хотите удалить этот чат?",
+    "limit_reached": "Достигнут лимит чатов",
+    "streaming": "ИИ думает..."
+  },
+  "admin": {
+    "context_editor": "Редактор основного контекста",
+    "context_placeholder": "Введите системный промпт для ИИ-ассистента...",
+    "save": "Сохранить",
+    "saved": "Сохранено",
+    "unsaved_changes": "Несохранённые изменения",
+    "version": "Версия",
+    "characters": "символов"
+  },
   "common": {
     "loading": "Загрузка...",
     "error": "Произошла ошибка",