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/cache/filecache.go

@@ -0,0 +1,209 @@
+// Package cache implements a Drive-modifiedTime-gated JSON file cache,
+// mirroring scripts/cache_utils.py.
+package cache
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"fuj-management/go/internal/io/drive"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+// DriveClient is the subset of drive.Client used by FileCache.
+type DriveClient interface {
+	ModifiedTime(ctx context.Context, fileID string) (string, error)
+}
+
+type cacheFile struct {
+	ModifiedTime string          `json:"modifiedTime"`
+	Data         json.RawMessage `json:"data"`
+	CachedAt     string          `json:"cachedAt"`
+}
+
+// FileCache wraps a Drive client to gate JSON file caching on sheet modifiedTime.
+//
+// Two TTL knobs mirror scripts/cache_utils.py:
+//   - ttl: if the cache file on disk is younger than this, skip the Drive check entirely.
+//   - apiCheckTTL: debounces in-memory Drive API calls per sheet ID.
+//
+// Atomic writes: data is marshaled to a .tmp file then os.Rename'd.
+// Cache files are compatible with Python's format:
+//
+//	{"modifiedTime":"…","data":…,"cachedAt":"…"}
+type FileCache struct {
+	drive       DriveClient
+	dir         string
+	sheetMap    map[string]string // cache key → Drive file ID
+	ttl         time.Duration
+	apiCheckTTL time.Duration
+	mu          sync.Mutex
+	lastChecked map[string]time.Time
+}
+
+// New creates a FileCache.
+// sheetMap maps cache keys to Google Sheets/Drive file IDs (mirrors CACHE_SHEET_MAP in config).
+func New(d DriveClient, dir string, sheetMap map[string]string, ttl, apiCheckTTL time.Duration) *FileCache {
+	return &FileCache{
+		drive:       d,
+		dir:         dir,
+		sheetMap:    sheetMap,
+		ttl:         ttl,
+		apiCheckTTL: apiCheckTTL,
+		lastChecked: make(map[string]time.Time),
+	}
+}
+
+// Get returns the cached value for cacheKey, calling fetch if the cache is stale.
+// T must be JSON-marshalable.
+func Get[T any](ctx context.Context, fc *FileCache, cacheKey string, fetch func(context.Context) (T, error)) (T, error) {
+	sheetID := fc.sheetMap[cacheKey]
+	if sheetID == "" {
+		sheetID = cacheKey
+	}
+	cacheFilePath := filepath.Join(fc.dir, cacheKey+"_cache.json")
+
+	currentModTime, err := fc.currentModifiedTime(ctx, sheetID, cacheFilePath)
+	if err != nil {
+		return *new(T), fmt.Errorf("cache: modifiedTime for %s: %w", cacheKey, err)
+	}
+
+	// Try cache hit
+	if data, ok := readCache[T](cacheFilePath, currentModTime); ok {
+		return data, nil
+	}
+
+	// Cache miss — fetch fresh data
+	fresh, err := fetch(ctx)
+	if err != nil {
+		return *new(T), err
+	}
+	if err := writeCache(cacheFilePath, currentModTime, fresh); err != nil {
+		// Non-fatal: log but don't fail the request
+		_, _ = fmt.Fprintf(os.Stderr, "cache: write %s: %v\n", cacheKey, err)
+	}
+	return fresh, nil
+}
+
+// Flush deletes all *_cache.json files in the cache dir and resets in-memory state.
+func (fc *FileCache) Flush() (int, error) {
+	fc.mu.Lock()
+	fc.lastChecked = make(map[string]time.Time)
+	fc.mu.Unlock()
+
+	pattern := filepath.Join(fc.dir, "*_cache.json")
+	matches, err := filepath.Glob(pattern)
+	if err != nil {
+		return 0, err
+	}
+	for _, f := range matches {
+		_ = os.Remove(f)
+	}
+	return len(matches), nil
+}
+
+// currentModifiedTime returns a stable string representing the current version
+// of the sheet, using the in-memory + file-mtime TTL guards before hitting Drive.
+// On Drive failure, falls back to a 5-minute bucket string (matching Python).
+func (fc *FileCache) currentModifiedTime(ctx context.Context, sheetID, cacheFilePath string) (string, error) {
+	now := time.Now()
+
+	fc.mu.Lock()
+	lastCheck := fc.lastChecked[sheetID]
+	fc.mu.Unlock()
+
+	// Guard 1: in-memory debounce — skip Drive if checked recently
+	if fc.apiCheckTTL > 0 && now.Sub(lastCheck) < fc.apiCheckTTL {
+		if mt, ok := readModifiedTime(cacheFilePath); ok {
+			return mt, nil
+		}
+	}
+
+	// Guard 2: cache file is young enough — trust the stored modifiedTime
+	if fc.ttl > 0 {
+		if info, err := os.Stat(cacheFilePath); err == nil {
+			if now.Sub(info.ModTime()) < fc.ttl {
+				if mt, ok := readModifiedTime(cacheFilePath); ok {
+					fc.mu.Lock()
+					fc.lastChecked[sheetID] = now
+					fc.mu.Unlock()
+					return mt, nil
+				}
+			}
+		}
+	}
+
+	// Hit Drive API
+	mt, err := fc.drive.ModifiedTime(ctx, sheetID)
+	if err != nil {
+		// Fallback: 5-minute bucket string, matches Python _fallback_ttl()
+		bucket := time.Now().Unix() / 300
+		return fmt.Sprintf("ttl-5m-%d", bucket), nil
+	}
+	fc.mu.Lock()
+	fc.lastChecked[sheetID] = now
+	fc.mu.Unlock()
+	return mt, nil
+}
+
+func readModifiedTime(path string) (string, bool) {
+	cf, ok := readCacheFile(path)
+	if !ok {
+		return "", false
+	}
+	return cf.ModifiedTime, cf.ModifiedTime != ""
+}
+
+func readCacheFile(path string) (cacheFile, bool) {
+	b, err := os.ReadFile(path)
+	if err != nil {
+		return cacheFile{}, false
+	}
+	var cf cacheFile
+	if err := json.Unmarshal(b, &cf); err != nil {
+		return cacheFile{}, false
+	}
+	return cf, true
+}
+
+func readCache[T any](path, currentModTime string) (T, bool) {
+	cf, ok := readCacheFile(path)
+	if !ok || cf.ModifiedTime != currentModTime {
+		return *new(T), false
+	}
+	var v T
+	if err := json.Unmarshal(cf.Data, &v); err != nil {
+		return *new(T), false
+	}
+	return v, true
+}
+
+func writeCache(path, modTime string, data any) error {
+	raw, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	cf := cacheFile{
+		ModifiedTime: modTime,
+		Data:         json.RawMessage(raw),
+		CachedAt:     time.Now().Format(time.RFC3339),
+	}
+	b, err := json.Marshal(cf)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+	tmp := path + ".tmp"
+	if err := os.WriteFile(tmp, b, 0o600); err != nil {
+		return err
+	}
+	return os.Rename(tmp, path)
+}
+
+// Ensure *drive.Client satisfies DriveClient at compile time.
+var _ DriveClient = (*drive.Client)(nil)