The five percent tier is lost at exactly one hundred dollars

The catalogue is a Flask service in src/bookshop. Prices are computed by

price_basket in src/bookshop/pricing.py, and the rules it follows are

documented in that module's docstring. Read them before touching anything.

The defect is in the tier discount. A basket whose subtotal is exactly

10 000 cents should earn the 5 % tier, and the same holds for 50 000

(10 %) and 200 000 (15 %). Today, a basket at exactly those amounts is

charged as if it were one cent below the threshold.

What to check

• discount_tier in src/bookshop/pricing.py returns the tier for a subtotal.
• A subtotal of exactly 10 000 must return the 5 % tier, not None.
• The same applies at 50 000 and 200 000.
• The rest of the pricing pipeline (voucher, tax, total) must keep producing

the same numbers it produced before.

Boundaries

• Do not change the tier thresholds, the tax rate, or the rounding rule.
• Do not modify the repository test suite under tests/.
• The public API of the bookshop package stays as it is.
• The repository's own test suite has to stay green after your change.