Here's a `main.py` file for a FastAPI backend named 'blog_app' with necessary imports and middleware, including SQLite and SQLAlchemy support:

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

app = FastAPI()

origins = [
    "http://localhost",
    "http://localhost:8000",
    "http://localhost:3000",

app.add_middleware(
    CORSMiddleware,
    allow_origins=origins,
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],

SQLALCHEMY_DATABASE_URL = "sqlite:///./blog_app.db"
engine = create_engine(SQLALCHEMY_DATABASE_URL, connect_args={"check_same_thread": False})
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)
Base = declarative_base()

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()


if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="0.0.0.0", port=8000)

This `main.py` file sets up the FastAPI application, configures CORS middleware, and establishes a connection to a SQLite database using SQLAlchemy. Here's a breakdown of the code:

4. Define a dependency function `get_db()` to provide a database session for each request.

Note: You'll need to install the required dependencies (`fastapi`, `uvicorn`, and `sqlalchemy`) before running this code. Additionally, you'll need to create your API routes, models, and other necessary components in separate files and import them as indicated in the placeholder comment.