fix: correct DeepSeek peak-window comment to weekdays only

[Frankie-Xu]

Summary

• Comment-only correction in packages/shared-python/shared/services/ai/token_costing.py: DeepSeek peak windows (01:00-04:00 & 06:00-10:00 UTC) apply Monday through Friday only; the whole weekend is off-peak (35 h/week, not 49).
• No API, worker, deployment, or migration impact. No numeric rates, model names, public APIs, env defaults, or database models were changed.
• Closes DeepSeek peak-window comment is missing "Monday through Friday" (35 h/week, not 49) #316

Verification

• uv sync --locked --all-packages --all-groups --python 3.11 (CPython 3.11.15)
• make lint — All checks passed
• make typecheck — 0 errors, 0 warnings, 0 informations
• uv run pytest packages/shared-python/shared/tests -q — 57 passed
• No manual API, worker, or local-dev checks (comment-only)

First full pytest run had 1 failure in test_page_memory_vlm_limiter_waits_then_succeeds (1s Redis wait timeout). Isolated re-run passed; a second full suite run was 57 passed. That test is unrelated to this comment change.

Deployment Notes

• No new or changed environment variables
• No database migrations, queue changes, storage changes, or release-order requirements
• Backwards compatible; rollback is git revert of this commit
• 无行为变化因此未跑集成测试

Checklist

• Tests were added or updated when behavior changed
• Public docs, examples, or OpenAPI contracts were updated when needed
• Database migrations are idempotent and safe to deploy
• Logs, errors, and validation paths avoid leaking secrets or user data
• The pull request description explains any breaking or user-visible change

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