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()
+}