Add first-class location and online_url for hybrid events

[edalzell]

Summary

• Declare location, online_url, and coordinates on the event fieldset so physical, online, and hybrid events are first-class
• Replace address/link URL sniffing with explicit ICS mapping (decorate / icsLocation / icsUrl), including the multi-day whole-event download gap
• Add a 7.0 MigrateLocationFields update script plus docs/tests for the mapping table and migration edge cases

Test plan

• vendor/bin/pest
• Create physical-only, online-only, and hybrid events; download ICS and verify LOCATION: / URL: / GEO:
• Confirm online-only join link is clickable in Calendar.app
• Download a multi-day event without a date param and confirm location/URL/description are present
• Run the 7.0 update path against sample address / link / URL-location entries (and confirm array-shaped location is left alone)

[edalzell]

Re-cutting into the stepped PRs from #190 (#191–#196) instead of one monolithic change. Closing this draft; work continues on those issues.