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>

backend/app/config.py

@@ -12,6 +12,9 @@ class Settings(BaseSettings):
 
     BACKEND_CORS_ORIGINS: list[str] = ["http://localhost", "http://localhost:3000"]
 
+    ANTHROPIC_API_KEY: str = ""
+    CLAUDE_MODEL: str = "claude-sonnet-4-20250514"
+
     FIRST_ADMIN_EMAIL: str = "admin@example.com"
     FIRST_ADMIN_USERNAME: str = "admin"
     FIRST_ADMIN_PASSWORD: str = "changeme_admin_password"