Refactor attendance logic into service layer

[LoopyB]

Summary

Reimplements the useful attendance refactor from PR #361 against current main.

Changes

• Added attendance.service.js
• Moved check-in, check-out, assignment validation, duplicate detection, location validation, and attendance history lookup out of the controller
• Updated attendance handling to use the current schema fields:
  • guardId
  • shiftId
  • checkInTime
  • checkOutTime
  • siteLocation.coordinates
• Preserved assignment behaviour through acceptedBy / assignedGuard
• Added controlled handling for invalid shift IDs
• Kept the controller thin by delegating attendance logic to the service layer

Validation

• Targeted ESLint passed
• Attendance Jest suite passed: 7 tests
• Backend started successfully
• GET /api/v1/health returned HTTP 200
• git diff --check passed

Scope

No payroll, seed-data, emergency, frontend, or Guard App files were changed.

Supersedes PR #361.