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/services/context_service.py

@@ -0,0 +1,44 @@
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.context_file import ContextFile
+
+DEFAULT_SYSTEM_PROMPT = """You are a personal AI health assistant. Your role is to:
+- Help users understand their health data and medical documents
+- Provide health-related recommendations based on uploaded information
+- Schedule reminders for checkups, medications, and health-related activities
+- Compile health summaries when requested
+- Answer health questions clearly and compassionately
+
+Always be empathetic, accurate, and clear. When uncertain, recommend consulting a healthcare professional.
+You can communicate in English and Russian based on the user's preference."""
+
+
+async def get_primary_context(db: AsyncSession) -> ContextFile | None:
+    result = await db.execute(
+        select(ContextFile).where(ContextFile.type == "primary", ContextFile.user_id.is_(None))
+    )
+    return result.scalar_one_or_none()
+
+
+async def upsert_primary_context(
+    db: AsyncSession, content: str, admin_user_id: uuid.UUID
+) -> ContextFile:
+    ctx = await get_primary_context(db)
+    if ctx:
+        ctx.content = content
+        ctx.version = ctx.version + 1
+        ctx.updated_by = admin_user_id
+    else:
+        ctx = ContextFile(
+            type="primary",
+            user_id=None,
+            content=content,
+            version=1,
+            updated_by=admin_user_id,
+        )
+        db.add(ctx)
+    await db.flush()
+    return ctx