Description

Layer 1 of the self sign-in stack. Adds Attendance.sign_in_method so attendance records how a member signed in, with a backfill to staff_initiated for existing rows. Today only staff-initiated sign-ins happen, so nothing downstream reads this yet; it exists so self sign-in can be told apart in attendance exports.

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 -- schema groundwork only, no user-facing behavior. The feature that consumes it (SELF_CHECK_IN) is gated in #122.