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```python
# AI Dev Tool: FastAPI Pydantic v2 Micro-SaaS Template
# Description: A production-ready FastAPI and Pydantic v2 template for building micro-SaaS applications
# Author: [Your Name]

from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel, Field
from pydantic.validators import UUID4
from typing import Optional
from datetime import datetime, timedelta
from jose import jwt, JWTError
from passlib.context import CryptContext
from sqlalchemy import create_engine, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

# Define the database connection
SQLALCHEMY_DATABASE_URL = "sqlite:///./sqlalchemy.db"

engine = create_engine(SQLALCHEMY_DATABASE_URL)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

Base = declarative_base()

# Define the user model
class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True, index=True)
    username = Column(String, unique=True, index=True)
    hashed_password = Column(String)

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