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```python
# AI Dev Tool: FastAPI Pydantic v2 Micro-SaaS Boilerplate
# Description: Production-ready FastAPI boilerplate with Pydantic v2 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 import validator
from typing import Optional
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, String
from sqlalchemy.exc import IntegrityError

# Initialize FastAPI application
app = FastAPI()

# Define OAuth2 scheme
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

# Define database connection
SQLALCHEMY_DATABASE_URL = "sqlite:///example.db"
engine = create_engine(SQLALCHEMY_DATABASE_URL)
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()

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

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