feat(go): IO layer behind interfaces (M4)

- io/attendance: CSV-over-public-URL client + Fake for adult/junior tabs
- io/drive: Drive v3 modifiedTime client + Fake
- io/sheets: Sheets v4 client (GetValues/AppendValues/BatchUpdateValues/
  WriteHeader/SortByDateColumn) + Fake with call-capture
- io/cache: Drive-modifiedTime-gated FileCache; two TTL knobs; atomic
  writes; generic Get[T]; Python-compatible JSON format; Flush()
- io/fio: Client interface backed by Fio REST API (apiClient) and HTML
  scraper (transparentClient); Fake; testdata fixtures
- membership/sources: NewSources wires attendance CSV + Sheets + cache
  into LoadAdults/LoadJuniors/LoadTransactions/LoadExceptions; Czech
  month parsing + merged-month maps
- banksync: SyncToSheets (SHA-256 dedup, optional sort) and
  InferPayments ([?] review prefix, dry-run) — tested with fakes
- cmd/fuj: sync and infer subcommands wired; fees and reconcile use
  real NewSources; go.mod gains google.golang.org/api + x/net
- gofumpt extra-rules applied across all packages; lint clean

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/services/membership/loader.go

@@ -9,10 +9,10 @@ import (
 // ErrIOPending is returned by stub loader methods until the M4 IO layer lands.
 var ErrIOPending = errors.New("io layer not yet wired up; lands in milestone M4 (sheets/drive/fio)")
 
-// AttendanceLoader loads processed adult attendance + computed fees from the
-// attendance Google Sheet.
+// AttendanceLoader loads attendance and computed fees from the attendance Google Sheet.
 type AttendanceLoader interface {
 	LoadAdults(ctx context.Context) (members []reconcile.Member, sortedMonths []string, err error)
+	LoadJuniors(ctx context.Context) (members []reconcile.Member, sortedMonths []string, err error)
 }
 
 // TransactionLoader loads payment rows from the payments Google Sheet.
@@ -41,6 +41,10 @@ func (stubSources) LoadAdults(_ context.Context) ([]reconcile.Member, []string,
 	return nil, nil, ErrIOPending
 }
 
+func (stubSources) LoadJuniors(_ context.Context) ([]reconcile.Member, []string, error) {
+	return nil, nil, ErrIOPending
+}
+
 func (stubSources) LoadTransactions(_ context.Context) ([]reconcile.Transaction, error) {
 	return nil, ErrIOPending
 }