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/services/banksync/sync.go

@@ -0,0 +1,139 @@
+// Package banksync implements the bank-sync and payment-inference operations.
+package banksync
+
+import (
+	"context"
+	"fmt"
+	"fuj-management/go/internal/domain/synch"
+	"fuj-management/go/internal/io/fio"
+	"strings"
+	"time"
+)
+
+// columnLabels is the canonical header for the payments sheet.
+// Mirrors COLUMN_LABELS in scripts/sync_fio_to_sheets.py.
+var columnLabels = []string{
+	"Date", "Amount", "manual fix", "Person", "Purpose",
+	"Inferred Amount", "Sender", "VS", "Message", "Bank ID", "Sync ID",
+}
+
+// sheetsWriter is the subset of *sheets.Client used by SyncToSheets.
+type sheetsWriter interface {
+	GetValues(ctx context.Context, spreadsheetID, a1Range string) ([][]any, error)
+	AppendValues(ctx context.Context, spreadsheetID, a1Range string, rows [][]any) error
+	WriteHeader(ctx context.Context, spreadsheetID string, labels []string) error
+	SortByDateColumn(ctx context.Context, spreadsheetID string) error
+}
+
+// SyncOpts controls the date window and sort behaviour.
+type SyncOpts struct {
+	Days     int       // look-back window when From/To are zero
+	From, To time.Time // explicit window (overrides Days)
+	Sort     bool      // sort the sheet by Date after appending
+}
+
+// SyncToSheets fetches Fio transactions and appends new ones to the payments sheet.
+// Returns the number of rows appended.
+// Ports scripts/sync_fio_to_sheets.py sync_to_sheets.
+func SyncToSheets(
+	ctx context.Context,
+	spreadsheetID string,
+	fioClient fio.Client,
+	sh sheetsWriter,
+	opts SyncOpts,
+) (int, error) {
+	// 1. Read existing rows to collect known Sync IDs (column K, index 10).
+	rows, err := sh.GetValues(ctx, spreadsheetID, "A1:K")
+	if err != nil {
+		return 0, fmt.Errorf("sync: read sheet: %w", err)
+	}
+
+	existingIDs := make(map[string]bool)
+	if len(rows) > 0 {
+		header := rows[0]
+		if !headerMatches(header) {
+			if err := sh.WriteHeader(ctx, spreadsheetID, columnLabels); err != nil {
+				return 0, fmt.Errorf("sync: write header: %w", err)
+			}
+		} else {
+			for _, row := range rows[1:] {
+				if len(row) > 10 {
+					if id, ok := row[10].(string); ok && id != "" {
+						existingIDs[id] = true
+					}
+				}
+			}
+		}
+	}
+
+	// 2. Compute date window.
+	from, to := opts.From, opts.To
+	if from.IsZero() || to.IsZero() {
+		to = time.Now()
+		days := opts.Days
+		if days <= 0 {
+			days = 30
+		}
+		from = to.AddDate(0, 0, -days)
+	}
+
+	// 3. Fetch Fio transactions.
+	txns, err := fioClient.FetchTransactions(ctx, from, to)
+	if err != nil {
+		return 0, fmt.Errorf("sync: fetch fio: %w", err)
+	}
+
+	// 4. Append new rows.
+	var newRows [][]any
+	for _, tx := range txns {
+		currency := tx.Currency
+		if currency == "" {
+			currency = "CZK"
+		}
+		id := synch.GenerateSyncID(synch.Transaction{
+			Date:     tx.Date,
+			Amount:   tx.Amount,
+			Currency: currency,
+			Sender:   tx.Sender,
+			VS:       tx.VS,
+			Message:  tx.Message,
+			BankID:   tx.BankID,
+		})
+		if existingIDs[id] {
+			continue
+		}
+		newRows = append(newRows, []any{
+			tx.Date, tx.Amount,
+			"", "", "", "", // manual fix, Person, Purpose, Inferred Amount
+			tx.Sender, tx.VS, tx.Message, tx.BankID, id,
+		})
+	}
+
+	if len(newRows) == 0 {
+		return 0, nil
+	}
+
+	if err := sh.AppendValues(ctx, spreadsheetID, "A2", newRows); err != nil {
+		return 0, fmt.Errorf("sync: append: %w", err)
+	}
+
+	if opts.Sort {
+		if err := sh.SortByDateColumn(ctx, spreadsheetID); err != nil {
+			return 0, fmt.Errorf("sync: sort: %w", err)
+		}
+	}
+	return len(newRows), nil
+}
+
+func headerMatches(row []any) bool {
+	if len(row) < len(columnLabels) {
+		return false
+	}
+	for i, label := range columnLabels {
+		cell, _ := row[i].(string)
+		if !strings.EqualFold(cell, label) {
+			return false
+		}
+	}
+	return true
+}