Description

Standardizes event check-in terminology as sign-in and moves the staff sign-in decision into a reusable helper.

• Renames the attendance method field and enum while preserving existing field values with a database column rename.
• Checks membership across all active terms, including overlaps and periods with no active term.
• Adds a migration regression test and retains staff sign-in coverage for term overlaps.

Layer 2 of the self sign-in stack.

Pre-Merge Checklist

• I've tested my code and solemnly swear that it works flawlessly
• For features, new functionality, or major refactors: My changes are gated behind feature flag(s).
  • N/A -- staff sign-in remains available, and the later self sign-in feature is separately gated.