docs: clarify DeepSeek V4 weekend pricing

[opencode-agent]

Summary

• clarify that DeepSeek V4 peak pricing applies only on weekdays (Beijing Time)
• document that Saturdays and Sundays are off-peak all day
• update both the Go and Zen pricing pages

Verification

• git diff --check
• bun run build (unavailable: Astro dependencies are not installed in this worktree)

Requested by: YIn via email

[xyzs996]

The weekday timezone is the part most docs get wrong, so it's good to see it spelled out here. One addition worth making: the sentence now mixes clocks — the hours are in UTC, the weekday is in Beijing time — and a reader in UTC still has to do the shift themselves to know when the weekend starts.

Adding the two edges in the same clock as the hours would close it:

Weekends (Saturday and Sunday, Beijing Time) are Off-Peak all day — that is, from 16:00 UTC Friday to 16:00 UTC Sunday.

Those are the only two instants where the Beijing and UTC calendars disagree in a way that changes the bill, and with the current windows nothing else in the schedule can reveal the difference: both peak windows sit clear of 16:00–24:00 UTC, so reading the weekday off the unshifted instant produces identical prices at all 168 hours of the week. Dated table of those edges if it's useful for tests: https://xyzs996.github.io/llm-api-pricing/deepseek-peak-hours.html