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/schemas/__init__.py

@@ -0,0 +1 @@
+"""Pydantic schemas for request/response validation"""

backend/app/schemas/user.py

@@ -0,0 +1,57 @@
+"""User schemas"""
+from pydantic import BaseModel, EmailStr, Field
+from typing import Optional
+from datetime import datetime
+
+
+class UserBase(BaseModel):
+    """Base user schema"""
+    username: str = Field(..., min_length=3, max_length=50)
+    email: EmailStr
+    full_name: Optional[str] = None
+
+
+class UserCreate(UserBase):
+    """User creation schema"""
+    password: str = Field(..., min_length=8)
+
+
+class UserLogin(BaseModel):
+    """User login schema"""
+    username: str
+    password: str
+
+
+class UserUpdate(BaseModel):
+    """User update schema"""
+    full_name: Optional[str] = None
+    bio: Optional[str] = None
+    avatar_url: Optional[str] = None
+
+
+class UserResponse(UserBase):
+    """User response schema"""
+    id: int
+    avatar_url: Optional[str] = None
+    bio: Optional[str] = None
+    is_active: bool
+    is_verified: bool
+    wallet_address: Optional[str] = None
+    balance: float
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class Token(BaseModel):
+    """JWT token response"""
+    access_token: str
+    refresh_token: str
+    token_type: str = "bearer"
+
+
+class TokenData(BaseModel):
+    """Token payload data"""
+    user_id: Optional[int] = None
+    username: Optional[str] = None