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/fio/fio_test.go

@@ -0,0 +1,157 @@
+package fio
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestAPIClient_ParseResponse(t *testing.T) {
+	body, err := os.ReadFile("testdata/api_response.json")
+	if err != nil {
+		t.Fatal(err)
+	}
+	txns, err := parseAPIResponse(body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(txns) != 1 {
+		t.Fatalf("want 1 txn (outgoing filtered), got %d", len(txns))
+	}
+	tx := txns[0]
+	if tx.Date != "2026-04-10" {
+		t.Errorf("date: want '2026-04-10', got %q", tx.Date)
+	}
+	if tx.Amount != 750 {
+		t.Errorf("amount: want 750, got %v", tx.Amount)
+	}
+	if tx.Sender != "Jana Novakova" {
+		t.Errorf("sender: want 'Jana Novakova', got %q", tx.Sender)
+	}
+	if tx.Message != "duben 2026" {
+		t.Errorf("message: want 'duben 2026', got %q", tx.Message)
+	}
+	if tx.VS != "123" {
+		t.Errorf("vs: want '123', got %q", tx.VS)
+	}
+	if tx.BankID != "12345678901" {
+		t.Errorf("bank_id: want '12345678901', got %q", tx.BankID)
+	}
+}
+
+func TestAPIClient_HTTPRoundTrip(t *testing.T) {
+	body, _ := os.ReadFile("testdata/api_response.json")
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write(body)
+	}))
+	defer srv.Close()
+
+	c := &apiClient{token: "TESTTOKEN", hc: &overrideClient{base: srv.Client(), baseURL: srv.URL}}
+	txns, err := c.FetchTransactions(context.Background(), time.Now().AddDate(0, -1, 0), time.Now())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(txns) != 1 {
+		t.Fatalf("want 1 txn, got %d", len(txns))
+	}
+}
+
+func TestTransparentClient_ParseHTML(t *testing.T) {
+	body, err := os.ReadFile("testdata/transparent.html")
+	if err != nil {
+		t.Fatal(err)
+	}
+	txns, err := parseTransparentHTML(body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Only the incoming row (750 CZK) should be kept; -200 is outgoing
+	if len(txns) != 1 {
+		t.Fatalf("want 1 txn (outgoing filtered), got %d", len(txns))
+	}
+	tx := txns[0]
+	if tx.Date != "2026-04-10" {
+		t.Errorf("date: want '2026-04-10', got %q", tx.Date)
+	}
+	if tx.Amount != 750 {
+		t.Errorf("amount: want 750, got %v", tx.Amount)
+	}
+	if tx.Sender != "Jana Novakova" {
+		t.Errorf("sender: want 'Jana Novakova', got %q", tx.Sender)
+	}
+	if tx.VS != "123" {
+		t.Errorf("vs: want '123', got %q", tx.VS)
+	}
+	if tx.BankID != "" {
+		t.Errorf("bank_id: want empty on HTML path, got %q", tx.BankID)
+	}
+}
+
+func TestParseCzechDate(t *testing.T) {
+	cases := []struct{ in, want string }{
+		{"10.04.2026", "2026-04-10"},
+		{"10/04/2026", "2026-04-10"},
+		{"", ""},
+		{"invalid", ""},
+	}
+	for _, c := range cases {
+		if got := parseCzechDate(c.in); got != c.want {
+			t.Errorf("parseCzechDate(%q) = %q, want %q", c.in, got, c.want)
+		}
+	}
+}
+
+func TestParseCzechAmount(t *testing.T) {
+	cases := []struct {
+		in   string
+		want float64
+	}{
+		{"750,00 CZK", 750},
+		{"1.500,00", 1500},
+		{"1500.00", 1500},
+		{"-200,00 CZK", -200},
+	}
+	for _, c := range cases {
+		if got := parseCzechAmount(c.in); got != c.want {
+			t.Errorf("parseCzechAmount(%q) = %v, want %v", c.in, got, c.want)
+		}
+	}
+}
+
+func TestFake(t *testing.T) {
+	f := &Fake{Transactions: []Transaction{{Date: "2026-04-01", Amount: 500}}}
+	txns, err := f.FetchTransactions(context.Background(), time.Now(), time.Now())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(txns) != 1 || txns[0].Date != "2026-04-01" {
+		t.Errorf("unexpected: %v", txns)
+	}
+}
+
+// overrideClient replaces the URL in requests so we can hit a local test server
+// instead of the real Fio URL.
+type overrideClient struct {
+	base    *http.Client
+	baseURL string
+}
+
+func (o *overrideClient) Do(req *http.Request) (*http.Response, error) {
+	r2, _ := http.NewRequestWithContext(req.Context(), req.Method, o.baseURL+req.URL.Path, nil)
+	resp, err := o.base.Do(r2)
+	if err != nil {
+		return nil, err
+	}
+	// The api client reads the body, so re-serve whatever the test server returned.
+	return resp, nil
+}
+
+// verify Fake satisfies Client
+var _ Client = (*Fake)(nil)
+
+// ensure io.ReadAll isn't called at top level (compile-time reference suppressor)
+var _ = io.ReadAll