Add comprehensive FastAPI backend for BlackRoad OS

This commit adds a complete backend infrastructure with:

**Core Infrastructure:**
- FastAPI application with async/await support
- PostgreSQL database with SQLAlchemy ORM
- Redis caching layer
- JWT authentication and authorization
- Docker and Docker Compose configuration

**API Services:**
- Authentication API (register, login, JWT tokens)
- RoadMail API (email service with folders, send/receive)
- BlackRoad Social API (posts, comments, likes, follows)
- BlackStream API (video streaming with views/likes)
- File Storage API (file explorer with upload/download)
- RoadCoin Blockchain API (mining, transactions, wallet)
- AI Chat API (conversations with AI assistant)

**Database Models:**
- User accounts with wallet integration
- Email and folder management
- Social media posts and engagement
- Video metadata and analytics
- File storage with sharing
- Blockchain blocks and transactions
- AI conversation history

**Features:**
- Complete CRUD operations for all services
- Real-time blockchain mining with proof-of-work
- Transaction validation and wallet management
- File upload with S3 integration (ready)
- Social feed with engagement metrics
- Email system with threading support
- AI chat with conversation persistence

**Documentation:**
- Comprehensive README with setup instructions
- API documentation (Swagger/ReDoc auto-generated)
- Deployment guide for multiple platforms
- Testing framework with pytest

**DevOps:**
- Docker containerization
- Docker Compose for local development
- Database migrations with Alembic
- Health check endpoints
- Makefile for common tasks

All APIs are production-ready with proper error handling,
input validation, and security measures.

backend/app/models/ai_chat.py

@@ -0,0 +1,56 @@
+"""AI Chat models"""
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, Enum
+from sqlalchemy.sql import func
+import enum
+from app.database import Base
+
+
+class MessageRole(str, enum.Enum):
+    """Message role types"""
+    USER = "user"
+    ASSISTANT = "assistant"
+    SYSTEM = "system"
+
+
+class Conversation(Base):
+    """AI conversation model"""
+
+    __tablename__ = "conversations"
+
+    id = Column(Integer, primary_key=True, index=True)
+    user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
+
+    title = Column(String(255))
+    model = Column(String(100), default="gpt-3.5-turbo")
+
+    # Metadata
+    message_count = Column(Integer, default=0)
+    total_tokens = Column(Integer, default=0)
+
+    # Timestamps
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(DateTime(timezone=True), onupdate=func.now())
+
+
+class Message(Base):
+    """AI chat message model"""
+
+    __tablename__ = "messages"
+
+    id = Column(Integer, primary_key=True, index=True)
+    conversation_id = Column(Integer, ForeignKey("conversations.id", ondelete="CASCADE"), nullable=False)
+
+    role = Column(Enum(MessageRole), nullable=False)
+    content = Column(Text, nullable=False)
+
+    # Token usage
+    tokens = Column(Integer)
+
+    # Metadata
+    model = Column(String(100))
+    finish_reason = Column(String(50))
+
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+
+    def __repr__(self):
+        return f"<Message {self.id} ({self.role})>"