fix(usage): respect server TZ for today filter and daily aggregation (#221)

The history endpoint parsed date parameters as UTC (using
`T00:00:00.000Z`) and grouped daily breakdowns via `.toISOString()`
(always UTC). For users outside UTC, this caused usage from the early
hours of their local day to be excluded from the "today" filter, and
daily chart labels to be off by one day during the UTC boundary overlap.

Now uses the server's local timezone (set via `TZ` env var) for both
date boundary calculation and daily aggregation key, so the dashboard
respects the configured timezone.