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>
ADULT_FEE_DEFAULT is 700 CZK, not 750. The 750 appears in
ADULT_FEE_MONTHLY_RATE for most current months but is not the fallback.
Rephrase the member-tiers bullet to point at the dict rather than a
number that drifts each season; update the fee-calc bullet to match
the junior line's style (default 700 vs default 500).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
