Recover automatic timesheet generation

[LoopyB]

Summary

Reimplements the useful automatic timesheet functionality from PR #372 against
the current attendance and payroll architecture.

Changes

• Added a Timesheet model keyed by shift and guard
• Added automatic timesheet generation for completed shifts with completed attendance
• Added idempotent generation to prevent duplicate timesheets
• Added employer, guard and admin retrieval scoping
• Reused the existing scheduled-hours and attendance-hours payroll helpers
• Added focused service tests

Endpoints

• POST /api/v1/timesheets/generate
• GET /api/v1/timesheets
• GET /api/v1/timesheets/:id

Compatibility with the current backend

• Uses guardId, shiftId, checkInTime and checkOutTime
• Does not port stale shift-completion hooks
• Does not store or calculate gross pay
• Does not introduce hardcoded pay assumptions
• Does not include frontend, Guard App, notification or socket changes

Validation

• Targeted ESLint passed
• Timesheet and payroll isolation tests passed
• 2 test suites passed
• 7 tests passed
• git diff --check passed

Scope

Only timesheet generation, retrieval and the export of existing payroll hour
helpers are included.

Supersedes PR #372.