User Registration and Profile API

Build a user registration and profile management API with FastAPI and Python.

Endpoints

• POST /api/users -- register a new user with username: str (3-20 chars, alphanumeric), email: str, password: str (min 8 chars), full_name: str
• GET /api/users/{user_id} -- get user profile (never return password)
• PATCH /api/users/{user_id} -- update user profile fields (partial update)
• DELETE /api/users/{user_id} -- soft-delete a user (set is_active = False)
• POST /api/users/{user_id}/change-password -- change password with old_password: str, new_password: str

Business rules:

• Username and email must be unique
• Cannot update or delete an already-deactivated user
• Old password must match when changing password

Output

Produce Python files:

• app/main.py -- FastAPI app with routes
• app/schemas.py -- Pydantic models for request/response bodies
• app/store.py -- in-memory user store (dict-based, no real database)

Do not include test files, authentication middleware, or deployment configuration.