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/config/config.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"strconv"
+	"strings"
 	"time"
 )
 
@@ -10,16 +11,34 @@ import (
 const (
 	AttendanceSheetID = "1E2e_gT_K5AwSRCDLDTa2UetZTkHmBOcz0kFbBUNUNBA"
 	PaymentsSheetID   = "1Om0YPoDVCH5cV8BrNz5LG5eR5MMU05ypQC7UMN1xn_Y"
-	JuniorSheetGID    = "1213318614"
+
+	// Both attendance tabs live in the same Google Spreadsheet (AttendanceSheetID).
+	// The original adult and junior attendance data lives in separate source spreadsheets,
+	// but is collected into this one sheet via IMPORTRANGE — one tab per group.
+	// Tabs are identified by the gid= query param in the CSV export URL.
+	AttendanceAdultSheetGID = "0"          // gid=0 — adult practices tab (IMPORTRANGE'd)
+	JuniorSheetGID          = "1213318614" // gid=1213318614 — junior practices tab (IMPORTRANGE'd)
 )
 
+// CacheSheetMap mirrors scripts/config.py CACHE_SHEET_MAP.
+// Maps a cache key to the Google Sheet ID whose Drive modifiedTime gates it.
+// Both attendance keys map to the same spreadsheet — different tabs, one Drive file.
+var CacheSheetMap = map[string]string{
+	"attendance_regular":    AttendanceSheetID,
+	"attendance_juniors":    AttendanceSheetID,
+	"exceptions_dict":       PaymentsSheetID,
+	"payments_transactions": PaymentsSheetID,
+}
+
 // Config holds all runtime configuration loaded from environment variables.
 // Mirrors scripts/config.py.
 type Config struct {
 	CredentialsPath  string
 	BankAccount      string
+	CacheDir         string
 	CacheTTL         time.Duration
 	CacheAPICheckTTL time.Duration
+	DriveTimeout     time.Duration
 	LogLevel         string
 	FioAPIToken      string
 	ServerAddr       string
@@ -31,14 +50,32 @@ func Load() Config {
 	return Config{
 		CredentialsPath:  env("CREDENTIALS_PATH", ".secret/fuj-management-bot-credentials.json"),
 		BankAccount:      env("BANK_ACCOUNT", "CZ8520100000002800359168"),
+		CacheDir:         env("CACHE_DIR", "tmp"),
 		CacheTTL:         envDuration("CACHE_TTL_SECONDS", 300),
 		CacheAPICheckTTL: envDuration("CACHE_API_CHECK_TTL_SECONDS", 300),
+		DriveTimeout:     envDuration("DRIVE_TIMEOUT_SECONDS", 10),
 		LogLevel:         env("LOG_LEVEL", "INFO"),
 		FioAPIToken:      env("FIO_API_TOKEN", ""),
 		ServerAddr:       env("SERVER_ADDR", ":8080"),
 	}
 }
 
+// IBANAccountNum extracts the bare account number from a Czech IBAN.
+// "CZ8520100000002800359168" → "2800359168"
+// Structure: CZ(2 check)(4 bank code)(16 zero-padded account).
+func IBANAccountNum(iban string) string {
+	s := strings.ReplaceAll(iban, " ", "")
+	if len(s) < 8 {
+		return iban
+	}
+	raw := s[8:] // 16-digit zero-padded account portion
+	n := strings.TrimLeft(raw, "0")
+	if n == "" {
+		return "0"
+	}
+	return n
+}
+
 func env(key, fallback string) string {
 	if v := os.Getenv(key); v != "" {
 		return v