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/schemas/chat.py

@@ -0,0 +1,62 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, Field
+
+
+class CreateChatRequest(BaseModel):
+    title: str | None = None
+
+
+class UpdateChatRequest(BaseModel):
+    title: str | None = None
+    is_archived: bool | None = None
+
+
+class SendMessageRequest(BaseModel):
+    content: str = Field(min_length=1, max_length=50000)
+
+
+class ChatResponse(BaseModel):
+    id: uuid.UUID
+    user_id: uuid.UUID
+    title: str
+    skill_id: uuid.UUID | None
+    is_archived: bool
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ChatListResponse(BaseModel):
+    chats: list[ChatResponse]
+
+
+class MessageResponse(BaseModel):
+    id: uuid.UUID
+    chat_id: uuid.UUID
+    role: str
+    content: str
+    metadata: dict | None = Field(None, alias="metadata_")
+    created_at: datetime
+
+    model_config = {"from_attributes": True, "populate_by_name": True}
+
+
+class MessageListResponse(BaseModel):
+    messages: list[MessageResponse]
+
+
+class ContextFileResponse(BaseModel):
+    id: uuid.UUID
+    type: str
+    content: str
+    version: int
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class UpdateContextRequest(BaseModel):
+    content: str