fix: restore resolution-specific date windows in SEO export script

fix: revert resolution-specific init dates, use uniform 6-year window

fix: restore per-resolution date ranges in export script

fix: use two-way date overlap for active station count (matches Shiny)

fix: align export active count with Shiny default date range (6 years)

feat: include activeStationCount in resolution-level SEO metadata

fix: align activeStationCount with Shiny app default date filters

feat: auto-adjust date range for historical stations and compute activeStationCount

feat: add DWD station SEO metadata and script to export data