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```python
# AI Dev Tool: FastAPI Pydantic v2 Micro-SaaS Starter
# ================================================
# A production-ready starter boilerplate for building micro-SaaS applications
# using FastAPI, Pydantic v2, and PostgreSQL Vector Search Extension.

from fastapi import FastAPI, Depends, HTTPException
from fastapi.security import OAuth2PasswordBearer, OAuth2PasswordRequestForm
from pydantic import BaseModel, Field
from typing import Optional
from psycopg2 import connect
from psycopg2.extras import DictCursor
from psycopg2.errors import UniqueViolation

# Initialize FastAPI application
app = FastAPI()

# OAuth2 password bearer
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="token")

# PostgreSQL database connection settings
DB_HOST = "localhost"
DB_NAME = "mydatabase"
DB_USER = "myuser"
DB_PASSWORD = "mypassword"

# Define Pydantic models
class User(BaseModel):
    """User model"""
    id: int = Field(..., description="User ID")
    username: str = Field(..., description="Username")
    email: str = Field(..., description="Email")
    password: str = Field(..., description="Password")

class Item(BaseModel):
    """Item

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