Money: Rename cents to minor units, ISO 4217 exponent-aware formatting

[sklinkert]

What & why

"Cents" is only the correct word for currencies with two decimal places. ISO 4217 defines a minor-unit exponent per currency: JPY, KRW, CLP and ISK have 0 (there is no sub-yen), BHD, JOD, KWD, OMR and TND have 3. A field named cents invites whoever adds JPY to the whitelist to divide by 100 and turn ¥5000 into ¥50.

This came directly out of reader feedback on the Money article. Changes:

• Money.cents → Money.minorUnits, accessor Cents() → MinorUnits(). No deprecated alias: clean break, all callers updated.
• The currency whitelist now carries each currency's ISO 4217 exponent, with the non-obvious exponents documented next to it. Adding a currency forces looking up the right exponent.
• String() formats from the exponent table instead of a hardcoded 100. New test pins JPY-style (exponent 0) and BHD-style (exponent 3) formatting.
• Wire format: price_cents → price_minor_units (JSON, DTOs, OpenAPI spec). Breaking API change, intentional — the template has no API consumers to migrate.
• Migration 000004 renames the DB column price_cents → price_minor_units; sqlc regenerated.
• Tutorial chapter 3, READMEs (EN + zh-CN) and code excerpts updated to match.

How to test

make test    # full suite incl. testcontainers — passes
make lint    # 0 issues
mkdocs build --strict   # docs still build clean

The new TestMoney_String_ExponentAware covers the exponent-0 and exponent-3 formatting paths.

Checklist

• make test passes (needs Docker)
• make lint passes
• Schema changes include up + down migrations and regenerated sqlc code
• New behavior is covered by tests