#179: Add date filter support for volunteer activity trend and volunteers by location

[Gyashaswi]

Closes #179

Summary

Added date filter support for two analytics widgets on the Super Admin dashboard.

Changes

• Added build_date_filter_trend() helper function
• Added build_date_filter_location() helper function
• Added get_grouping() helper function
• Updated get_volunteer_activity_trend() to support 7D, 30D, 1Y, All, Custom filters
• Updated get_volunteers_by_location() to support 7D, 30D, 1Y, All, Custom filters
• Renamed response key from month to period in trend data
• Trend and location filters are independent of each other
• Works for both Virginia and Ireland databases

Testing

• Tested locally using data-analytics/sql/ CSV files with all 5 filter scenarios (All, 7D, 30D, 1Y, Custom)
• All acceptance criteria verified

Notes

• Production credentials are not hardcoded
• Existing response structure, country aggregation logic, and error handling unchanged