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>

GeneralPlan.md

@@ -208,8 +208,8 @@ Daily scheduled job (APScheduler, 8 AM) reviews each user's memory + recent docs
 - Summary: Monorepo setup, Docker Compose, FastAPI + Alembic, auth (JWT), frontend shell (Vite + React + shadcn/ui + i18n), seed admin script
 
 ### Phase 2: Chat & AI Core
-- **Status**: NOT STARTED
-- [ ] Subplan created (`plans/phase-2-chat-ai.md`)
+- **Status**: IN PROGRESS
+- [x] Subplan created (`plans/phase-2-chat-ai.md`)
 - [ ] Phase completed
 - Summary: Chats + messages tables, chat CRUD, SSE streaming, Claude API integration, context assembly, frontend chat UI, admin context editor, chat limits