Build a personal expense tracker as a code-visible learning project

I'm a second-year CS student who's done a couple of Python courses but never built a full-stack app. Build a personal expense tracker web app with me — but the point of this project is that I understand every part of it, not just that it works, so build it in visible stages and explain the code as you go.

What to build:
- A Python and Flask backend with a plain HTML/JS frontend app where I can add an expense (amount, category, date, note), see a list of all expenses, filter by category and date range, and see a running total plus a simple bar chart of spend by category.
- Store data in a real database (SQLite is fine), not in-memory — I want to see an actual schema and real queries, not a JavaScript array pretending to be a database.

How to work with me — this is the important part:
1. Before writing code, show me the file structure and database schema you're about to create, and explain why it's shaped that way.
2. Build it in stages: (a) database and models, (b) the add-expense form and save logic, (c) the list and filter view, (d) the chart. Pause after each stage, show me the code, and give a 3-5 sentence plain-English explanation of what it does and why.
3. When you use a library or pattern I might not know — for example, an ORM like SQLAlchemy, or the fetch API for async requests — add a short comment in the code explaining it, not just the code itself.
4. Don't silently fix bugs. If something breaks, tell me what broke and why before you fix it.

Constraints:
- Keep the UI simple and functional — this project is about the code, not the design.
- I want to be able to explain this project in a job interview afterward, so favor clear, conventional code over clever one-liners.