Camptix: Add "Purchase Week" to the ticket summary options

[ryelle]

This adds a new summary option for the ticket purchase data, grouping by "purchase week". Showing purchases by date can be too detailed and hard to see trends when ticket sales are open for more than a few weeks, so showing by week can give data without requiring the organizer to download and graph the dates (which I've definitely done).

The only downside is the column title is just the week number, not very human friendly. I used this to check the weeks: https://www.epochconverter.com/weeks/2025

Screenshots

How to test the changes in this Pull Request:

1. Go to the summarize tab in Tickets > Tools
2. Select "Purchase week" to summarize by
3. Show summary
4. You should see the tickets purchased by week numbers

[dd32]

I wonder if it would be better to include a Date range or Year in the value? Something like Purchase Week: 2025 Week 50 (Dec 8-14) or Purchase Week: 2026 Week 1?

(Yeah I realise that maybe it's harder to implement)