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/api/v1/admin.py

@@ -0,0 +1,33 @@
+from typing import Annotated
+
+from fastapi import APIRouter, Depends
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.api.deps import require_admin
+from app.database import get_db
+from app.models.user import User
+from app.schemas.chat import ContextFileResponse, UpdateContextRequest
+from app.services import context_service
+
+router = APIRouter(prefix="/admin", tags=["admin"])
+
+
+@router.get("/context", response_model=ContextFileResponse | None)
+async def get_primary_context(
+    _admin: Annotated[User, Depends(require_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+):
+    ctx = await context_service.get_primary_context(db)
+    if not ctx:
+        return None
+    return ContextFileResponse.model_validate(ctx)
+
+
+@router.put("/context", response_model=ContextFileResponse)
+async def update_primary_context(
+    data: UpdateContextRequest,
+    admin: Annotated[User, Depends(require_admin)],
+    db: Annotated[AsyncSession, Depends(get_db)],
+):
+    ctx = await context_service.upsert_primary_context(db, data.content, admin.id)
+    return ContextFileResponse.model_validate(ctx)