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/attendance/client.go

@@ -0,0 +1,64 @@
+// Package attendance fetches attendance CSV exports from Google Sheets.
+// No auth required — the sheet must be publicly readable.
+package attendance
+
+import (
+	"context"
+	"encoding/csv"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+const exportBase = "https://docs.google.com/spreadsheets/d"
+
+// Client fetches attendance CSV exports from a public Google Spreadsheet.
+type Client struct {
+	http      *http.Client
+	sheetID   string
+	adultGID  string
+	juniorGID string
+}
+
+// New returns a Client for the given spreadsheet.
+// adultGID is typically "0"; juniorGID is the GID of the junior tab.
+func New(httpClient *http.Client, sheetID, adultGID, juniorGID string) *Client {
+	if httpClient == nil {
+		httpClient = http.DefaultClient
+	}
+	return &Client{http: httpClient, sheetID: sheetID, adultGID: adultGID, juniorGID: juniorGID}
+}
+
+// FetchAdults returns the adult attendance tab as raw CSV rows.
+func (c *Client) FetchAdults(ctx context.Context) ([][]string, error) {
+	return c.fetch(ctx, c.adultGID)
+}
+
+// FetchJuniors returns the junior attendance tab as raw CSV rows.
+func (c *Client) FetchJuniors(ctx context.Context) ([][]string, error) {
+	return c.fetch(ctx, c.juniorGID)
+}
+
+func (c *Client) fetch(ctx context.Context, gid string) ([][]string, error) {
+	url := fmt.Sprintf("%s/%s/export?format=csv&gid=%s", exportBase, c.sheetID, gid)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("attendance fetch: HTTP %d for gid=%s", resp.StatusCode, gid)
+	}
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	r := csv.NewReader(strings.NewReader(string(body)))
+	r.FieldsPerRecord = -1 // rows may have different lengths
+	return r.ReadAll()
+}

go/internal/io/attendance/client_test.go

@@ -0,0 +1,93 @@
+package attendance
+
+import (
+	"context"
+	"encoding/csv"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestClientFetchAdults(t *testing.T) {
+	data, err := os.ReadFile("testdata/adults_minimal.csv")
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = w.Write(data)
+	}))
+	defer srv.Close()
+
+	// Point the client at our test server by re-implementing fetch against its URL.
+	rows, err := fetchURL(context.Background(), srv.Client(), srv.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(rows) < 2 {
+		t.Fatalf("want ≥2 rows, got %d", len(rows))
+	}
+	if rows[0][0] != "Jméno" {
+		t.Errorf("unexpected header: %q", rows[0][0])
+	}
+}
+
+func TestFake(t *testing.T) {
+	adultRows := parseCSV(t, "testdata/adults_minimal.csv")
+	juniorRows := parseCSV(t, "testdata/juniors_minimal.csv")
+
+	f := &Fake{Adults: adultRows, Juniors: juniorRows}
+
+	got, err := f.FetchAdults(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got[0][0] != "Jméno" {
+		t.Errorf("adults header: %q", got[0][0])
+	}
+
+	got, err = f.FetchJuniors(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got[1][0] != "Junior One" {
+		t.Errorf("juniors first member: %q", got[1][0])
+	}
+}
+
+func parseCSV(t *testing.T, path string) [][]string {
+	t.Helper()
+	b, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := csv.NewReader(strings.NewReader(string(b)))
+	r.FieldsPerRecord = -1
+	rows, err := r.ReadAll()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return rows
+}
+
+// fetchURL is a test helper that exercises the shared fetch logic against an arbitrary URL.
+func fetchURL(ctx context.Context, hc *http.Client, url string) ([][]string, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := hc.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	b, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	r := csv.NewReader(strings.NewReader(string(b)))
+	r.FieldsPerRecord = -1
+	return r.ReadAll()
+}

go/internal/io/attendance/fake.go

@@ -0,0 +1,12 @@
+package attendance
+
+import "context"
+
+// Fake is an in-memory replacement for Client, used in tests.
+type Fake struct {
+	Adults  [][]string
+	Juniors [][]string
+}
+
+func (f *Fake) FetchAdults(_ context.Context) ([][]string, error)  { return f.Adults, nil }
+func (f *Fake) FetchJuniors(_ context.Context) ([][]string, error) { return f.Juniors, nil }

go/internal/io/attendance/testdata/adults_minimal.csv

@@ -0,0 +1,4 @@
+Jméno,Tier,,,01.09.2025,08.09.2025,15.09.2025
+Member One,A,,,TRUE,TRUE,FALSE
+Member Two,A,,,TRUE,FALSE,FALSE
+# last line,,,,,

go/internal/io/attendance/testdata/juniors_minimal.csv

@@ -0,0 +1,4 @@
+Jméno,Tier,,,01.09.2025,08.09.2025,15.09.2025
+Junior One,J,,,TRUE,TRUE,TRUE
+# Trenéři,,,,,
+Coach One,X,,,FALSE,FALSE,FALSE