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/io/drive/fake.go

@@ -0,0 +1,18 @@
+package drive
+
+import "context"
+
+// Fake is an in-memory replacement for Client used in tests.
+type Fake struct {
+	// Times maps file ID → modifiedTime string returned by ModifiedTime.
+	Times map[string]string
+	// Err, if non-nil, is returned instead of looking up Times.
+	Err error
+}
+
+func (f *Fake) ModifiedTime(_ context.Context, fileID string) (string, error) {
+	if f.Err != nil {
+		return "", f.Err
+	}
+	return f.Times[fileID], nil
+}