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/sheets/client.go

@@ -0,0 +1,124 @@
+// Package sheets provides a typed wrapper around the Google Sheets v4 API.
+package sheets
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"google.golang.org/api/option"
+	sheetsv4 "google.golang.org/api/sheets/v4"
+)
+
+// ValueRange pairs an R1C1 range with its cell values, used for batchUpdate.
+type ValueRange struct {
+	Range  string  // R1C1 notation, e.g. "R2C4:R2C6"
+	Values [][]any // one sub-slice per row
+}
+
+// Client wraps the Sheets v4 API with the operations needed by this project.
+type Client struct {
+	svc *sheetsv4.Service
+}
+
+// New builds a Client using a service-account credentials file.
+func New(ctx context.Context, credentialsPath string, _ time.Duration) (*Client, error) {
+	svc, err := sheetsv4.NewService(ctx,
+		option.WithCredentialsFile(credentialsPath), //nolint:staticcheck
+		option.WithScopes(sheetsv4.SpreadsheetsScope),
+	)
+	if err != nil {
+		return nil, err
+	}
+	return &Client{svc: svc}, nil
+}
+
+// GetValues fetches a range from a spreadsheet with UNFORMATTED_VALUE rendering
+// (numbers as numbers, dates as serial floats — matching Python's behaviour).
+func (c *Client) GetValues(ctx context.Context, spreadsheetID, a1Range string) ([][]any, error) {
+	resp, err := c.svc.Spreadsheets.Values.
+		Get(spreadsheetID, a1Range).
+		ValueRenderOption("UNFORMATTED_VALUE").
+		Context(ctx).
+		Do()
+	if err != nil {
+		return nil, err
+	}
+	rows := make([][]any, len(resp.Values))
+	copy(rows, resp.Values)
+	return rows, nil
+}
+
+// AppendValues appends rows to the first empty row after a1Range.
+func (c *Client) AppendValues(ctx context.Context, spreadsheetID, a1Range string, rows [][]any) error {
+	vals := make([][]any, len(rows))
+	copy(vals, rows)
+	_, err := c.svc.Spreadsheets.Values.
+		Append(spreadsheetID, a1Range, &sheetsv4.ValueRange{Values: vals}).
+		ValueInputOption("USER_ENTERED").
+		Context(ctx).
+		Do()
+	return err
+}
+
+// BatchUpdateValues writes multiple non-contiguous ranges in one API call.
+func (c *Client) BatchUpdateValues(ctx context.Context, spreadsheetID string, updates []ValueRange) error {
+	data := make([]*sheetsv4.ValueRange, len(updates))
+	for i, u := range updates {
+		vals := make([][]any, len(u.Values))
+		copy(vals, u.Values)
+		data[i] = &sheetsv4.ValueRange{Range: u.Range, Values: vals}
+	}
+	_, err := c.svc.Spreadsheets.Values.
+		BatchUpdate(spreadsheetID, &sheetsv4.BatchUpdateValuesRequest{
+			ValueInputOption: "USER_ENTERED",
+			Data:             data,
+		}).
+		Context(ctx).
+		Do()
+	return err
+}
+
+// WriteHeader overwrites row 1 of the spreadsheet with the given labels.
+func (c *Client) WriteHeader(ctx context.Context, spreadsheetID string, labels []string) error {
+	row := make([]any, len(labels))
+	for i, l := range labels {
+		row[i] = l
+	}
+	_, err := c.svc.Spreadsheets.Values.
+		Update(spreadsheetID, "A1", &sheetsv4.ValueRange{Values: [][]any{row}}).
+		ValueInputOption("USER_ENTERED").
+		Context(ctx).
+		Do()
+	return err
+}
+
+// SortByDateColumn sorts rows 2..10000 of the first sheet ascending by column A (Date).
+// Looks up the sheetId (gid) from spreadsheet metadata.
+func (c *Client) SortByDateColumn(ctx context.Context, spreadsheetID string) error {
+	meta, err := c.svc.Spreadsheets.Get(spreadsheetID).Context(ctx).Do()
+	if err != nil {
+		return fmt.Errorf("sheets: get spreadsheet: %w", err)
+	}
+	if len(meta.Sheets) == 0 {
+		return fmt.Errorf("sheets: spreadsheet has no sheets")
+	}
+	sheetID := meta.Sheets[0].Properties.SheetId
+
+	_, err = c.svc.Spreadsheets.BatchUpdate(spreadsheetID, &sheetsv4.BatchUpdateSpreadsheetRequest{
+		Requests: []*sheetsv4.Request{{
+			SortRange: &sheetsv4.SortRangeRequest{
+				Range: &sheetsv4.GridRange{
+					SheetId:       sheetID,
+					StartRowIndex: 1,
+					EndRowIndex:   10000,
+				},
+				SortSpecs: []*sheetsv4.SortSpec{{
+					DimensionIndex: 0,
+					SortOrder:      "ASCENDING",
+				}},
+			},
+		}},
+	}).Context(ctx).Do()
+	return err
+}