fix(billing): treat weekends and configured holidays as off-peak

[argszero]

Summary

billing::is_peak_hour() looked at the hour only, so every day of the week was billed at the DeepSeek peak price between 9:00-12:00 and 14:00-18:00 Beijing time. DeepSeek's own rule is narrower: peak is Monday to Friday, excluding Chinese statutory holidays; everything else — including weekends and holidays, all day — is off-peak, and the off-peak price is half the peak price. So deepseek-flash / deepseek-v4-pro calls on a Saturday or Sunday were charged double: the platform billed users more than the upstream rule justifies.

The weekday part is code; the statutory-holiday calendar is data, and it now comes from a new [billing] holidays table. The work order left that choice to the implementer and recommended configuration, because a table in code would force a release every year.

Rant: 2026-10-16T14:08:13.986531+08:00 — 【计费】高峰时段判定不看星期几 —— 周末按 2 倍价扣点，而 DeepSeek 官方在周末是空闲价 / "the peak-hour test ignores the day of the week — weekends are charged double, while DeepSeek charges the off-peak rate on weekends" (translated from the Chinese original).

Related Issue

No issue — the input is this project's own rant queue; the rant timestamp is named above.

Changes

• src/billing.rs — is_peak_hour(utc, holidays): a weekend returns off-peak; a date listed in holidays returns off-peak; the 9-12 / 14-18 window itself is unchanged. Doc comment updated.
• src/config.rs — new Billing { holidays: Vec<NaiveDate> }, optional ([billing] absent ⇒ empty table ⇒ weekends only), plus tests pinning the YYYY-MM-DD format, the empty default, and a malformed date being a startup error.
• src/gateway.rs — the single production call site passes &st.cfg.billing.holidays.
• config/config.example.toml — new [billing] section documenting the rule and the table, and the [[models]] note no longer says the peak window is Monday to Sunday.

⚠️ Operator note: config/config.toml is gitignored and is not part of this repository, so this PR ships the tracked example only. An instance with no [billing] section gets the weekend fix immediately and exempts no holidays until holidays is filled in from the official annual calendar. No holiday dates are invented here — the shipped table is empty on purpose.

Deliberately out of scope: the marketplace i18n strings (mk.peak.title, mk.detail.peakVal) name the peak window (9-12 / 14-18) and stay true; effective_prices()'s "peak_* = 0 means keep the off-peak price" semantics and every other pricing path are untouched.

Tests

• cargo test — 564 passed / 0 failed (562 on main; this PR adds 2 config tests and rewrites the peak-hour test).
• New legs: Monday 9:00 peak, 8:59 off, 12:00 off, 11:59 peak, 14:00 peak, 13:59 off, 18:00 off, 17:59 peak; Friday 10:00 peak; Monday 0:00 and 23:00 off; Saturday 10:00 off, Sunday 10:00 off; a listed holiday 10:00 off while its neighbouring weekday stays peak; and the empty table meaning "weekends only".
• Mutation check as the work order requires: with the weekday guard reduced to a never-matching arm, the Saturday leg fails at src/billing.rs:327; restoring the guard makes it pass.
• cargo fmt --check clean; cargo clippy --all-targets -- -D warnings clean.
• git grep 周一至周日 → zero hits in tracked files.

Checklist

• Branch name follows the convention (fix/…)
• Commit message uses Conventional Commits
• Configuration change mirrored in config/config.example.toml
• Single purpose, smallest reasonable diff