Feature 2 — Expense categories

Why

As a flatmate, I want each expense tagged with a category, so that at the end of the month I can see how much of our money went on rent, on groceries and on fun, instead of scrolling through thirty lines.

What

Every expense gets a category, chosen from a fixed list when the expense is created. The group page shows a total per category.

• [ ] logic.CATEGORIES is exactly the list ["rent", "groceries", "fun", "transport", "other"], in that order.
• [ ] expenses.csv has a new column category, and every expense (including the seed data) has a value from that list. In the seed, the dinner at Zé dos Cornos (group Jantar de Curso) is fun.
• [ ] add_expense(...) accepts a category argument and saves it with the expense.
• [ ] A new function category_totals(group_id) returns a dict {category: total} with the total amount spent per category in that group. Categories with no expenses do not appear.

Adding a column means touching three places: COLUMNS in db.py, the CSVs in seed/ (add the column and a category to every line), and then uv run python seed.py so that data/ gets the new column too. Do not edit data/ by hand.

Out of scope

• Changing the category of an existing expense.
• Custom categories; the list is fixed.
• Charts. A short list of "category: total" lines is enough.

Done when

• pytest tests/test_feature_2.py is green.
• The Add expense form has a category selectbox, and the group page shows a total per category.
• uv run python seed.py runs cleanly and the app still works after it.
• The work is on a branch called feature-2-categories and pushed.