Ports calculate_fee and calculate_junior_fee from scripts/attendance.py
into a new go/internal/domain/fees package. Introduces the Expected type
(Value int, Unknown bool) for the junior "?" sentinel, keeping the Go
API strictly typed instead of mirroring Python's str|int return.
All 20 table-driven tests pass with -race; golangci-lint clean.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
