Here's the `user.py` file for the `app/api/v1/models/` directory in the `blog_app` FastAPI backend:

from sqlalchemy import Column, ForeignKey, Integer, String, Text
from app.api.db.database import Base

class User(Base):
    __tablename__ = "user"

    id = Column(Integer, primary_key=True, index=True)
    username = Column(String, unique=True, nullable=False)
    email = Column(String, unique=True, nullable=False)
    password_hash = Column(String, nullable=False)
    full_name = Column(String)
    bio = Column(Text)
    profile_pic = Column(String)

Explanation:

1. We import the necessary classes from `sqlalchemy` for defining the database columns: `Column`, `ForeignKey`, `Integer`, `String`, and `Text`.
5. We define the following columns for the `User` model:
   - `id`: An integer primary key column with an index.
   - `username`: A string column that must be unique and cannot be null.
   - `email`: A string column that must be unique and cannot be null.
   - `password_hash`: A string column that cannot be null (for storing the hashed password).
   - `full_name`: An optional string column for the user's full name.
   - `bio`: An optional text column for the user's biography.
   - `profile_pic`: An optional string column for the user's profile picture URL.