DKRZ monthly and daily usage statistics

Files: dkrz-monthly.csv and dkrz-daily.csv

Format: UTF-8, semicolon delimiter, decimal point, six columns:
date;country;requests;downloads;downloads_size;visits

Both files start in January 2025. Each monthly row represents a calendar month;
its date is the first day of that month. Each daily row represents a calendar
day in UTC. DKRZ only; IPSL and CEDA are excluded. Both files are refreshed by
scripts/aggregate_dashboard_year.py --all-years. Their first and last rows
identify the current coverage. No extrapolation is applied. Earlier periods
remain available in the annual reports and their source CSVs.

All requests come from the Copernicus CDS, located in the UK (country code GB).
Visits are empty because distinct users cannot be determined from these
aggregated dashboards. Requests, downloads, and concurrency are not visits.

Monthly requests match dkrz_requests in roocs-<year>-monthly.csv and the annual
reports. Daily requests sum the success and failed series in the DKRZ
"Activity - Requests per day" chart. Both include successful and failed jobs.
Downloads are the sum of the DKRZ "Downloads per day" request_type series,
filtered to each reporting month. These are recorded fetch counts, not
distinct output filenames. Download counts need not equal request counts.
Monthly downloads_size matches dkrz_subsetted_data_gb in those annual-report
CSVs. Daily downloads_size uses the DKRZ "Data transfer per day (in GB)" chart.
Both use 1 GB = 1024 MB, consistent with the previous project exports, and
two decimal places. Monthly source sizes are independently rounded to 0.01 GB,
so summing daily sizes can differ from monthly sizes by a few MB.

Known source limitations:
- Some charts omit individual days. All calendar dates in each reporting month
  are included in the daily file, but missing metrics are empty rather than zero.
  A metric is only zero when the source chart explicitly records zero.
  Different metrics may have different daily coverage. Monthly totals reflect
  the available records and can therefore be incomplete.
- Daily count totals reconcile to the monthly counts for the current exports;
  this does not establish that the original source logs cover every day.
- Monthly exports are used throughout, as in the annual reports. Quarterly
  exports may differ from the sum of the monthly exports.

Sources in this repository:
- docs/downloads/stats/roocs-<year>-monthly.csv (2025 onward)
- docs/dashboard/summary-<year>.md (2025 onward)
- docs/downloads/dashboard/<year>/*dkrz*.html (monthly exports)