Feature 1 — Balances in plain words

Blocked by issue 001. The numbers are computed by compute_balances, which is wrong today. Fix the bug first, otherwise this feature will proudly show wrong balances.

Why

As a member of a group, I want to see at a glance who is owed money and who owes, in words I can read out loud to my flatmates, so that we stop arguing over a spreadsheet.

Today the group page has a "Balances" box that shows raw numbers next to member ids. Nobody knows who "9" is.

What

A new function balance_lines(group_id) in logic.py returns one sentence per member, in the same order as the members list. The group page shows those sentences instead of the raw numbers.

• [ ] balance_lines(group_id) returns exactly one line per member of the group.
• [ ] A member with a positive balance gets "<Name> is owed €<amount>", e.g. João is owed €20.00.
• [ ] A member with a negative balance gets "<Name> owes €<amount>", e.g. Carolina owes €10.00 (no minus sign).
• [ ] A member whose balance is zero gets "<Name> is settled up".
• [ ] Amounts always have two decimals (€8.20, not €8.2).

Out of scope

• Who should pay whom to settle everything (that is the Settle up section, later).
• Rounding the balances themselves (issue 003 is about shares, not balances).
• Changing the way balances are stored.

Done when

• pytest tests/test_feature_1.py is green (and the bug 001 test with it).
• The group page shows the sentences under the "Balances" heading, and the raw numbers are gone.
• The work is on a branch called feature-1-balances and pushed.