feat(go/M2.6): port domain/synch.GenerateSyncID

SHA-256 dedup hash from sync_fio_to_sheets.py generate_sync_id.
Key subtlety: Python str(float) emits "500.0" for whole-valued floats
and switches to scientific notation at |f|>=1e16 or |f|<1e-4 —
replicated via formatAmount using 'f'/'e' format selection.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/domain/synch/synch.go

@@ -0,0 +1,65 @@
+// Package synch ports the bank-sync deduplication helper from
+// scripts/sync_fio_to_sheets.py.
+package synch
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"math"
+	"strconv"
+	"strings"
+)
+
+// Transaction is the projection of a Fio transaction that participates
+// in the Sync ID hash. Other fields (ks, ss, sender_account, …) are
+// intentionally excluded — they are not part of the Python hash.
+//
+// Currency: leave "" to inherit the Python default of "CZK" (matches
+// the HTML scraper path which omits the key entirely).
+type Transaction struct {
+	Date     string
+	Amount   float64
+	Currency string
+	Sender   string
+	VS       string
+	Message  string
+	BankID   string
+}
+
+// GenerateSyncID returns the lowercase SHA-256 hex digest of
+// "date|amount|currency|sender|vs|message|bank_id" (lower-cased), used
+// as the dedup key in column K of the payments sheet.
+//
+// Byte-stable with scripts/sync_fio_to_sheets.py generate_sync_id.
+func GenerateSyncID(tx Transaction) string {
+	currency := tx.Currency
+	if currency == "" {
+		currency = "CZK"
+	}
+	raw := strings.ToLower(strings.Join([]string{
+		tx.Date,
+		formatAmount(tx.Amount),
+		currency,
+		tx.Sender,
+		tx.VS,
+		tx.Message,
+		tx.BankID,
+	}, "|"))
+	sum := sha256.Sum256([]byte(raw))
+	return hex.EncodeToString(sum[:])
+}
+
+// formatAmount mimics Python's str(float) for Fio transaction amounts.
+// Python uses decimal notation for abs(f) in [1e-4, 1e16) and scientific
+// notation outside that range, always adding ".0" to whole-valued decimals.
+func formatAmount(f float64) string {
+	abs := math.Abs(f)
+	if abs != 0 && (abs < 1e-4 || abs >= 1e16) {
+		return strconv.FormatFloat(f, 'e', -1, 64)
+	}
+	s := strconv.FormatFloat(f, 'f', -1, 64)
+	if !strings.ContainsRune(s, '.') {
+		s += ".0"
+	}
+	return s
+}

go/internal/domain/synch/synch_test.go

@@ -0,0 +1,119 @@
+package synch
+
+import (
+	"testing"
+)
+
+// All expected digests verified against the live Python implementation on 2026-05-06:
+//
+//	PYTHONPATH=scripts:. python -c '
+//	from sync_fio_to_sheets import generate_sync_id
+//	cases = [
+//	    {"date":"2026-01-15","amount":500.0,"currency":"CZK","sender":"Jan Novak","vs":"123","message":"clenske 1/2026","bank_id":"abc123"},
+//	    {"date":"2026-01-15","amount":500.0,"sender":"Jan Novak","vs":"123","message":"clenske 1/2026","bank_id":"abc123"},
+//	    {"date":"2026-02-10","amount":1234.56,"currency":"CZK","sender":"ABC SRO","vs":"","message":"FAKTURA 42","bank_id":"xyz"},
+//	    {"date":"2026-03-01","amount":-500.0,"currency":"CZK","sender":"refund","vs":"","message":"","bank_id":""},
+//	    {"date":"2026-04-01","amount":0.0,"currency":"CZK","sender":"","vs":"","message":"","bank_id":""},
+//	    {"date":"","amount":0.0,"currency":"CZK","sender":"","vs":"","message":"","bank_id":""},
+//	]
+//	for c in cases: print(generate_sync_id(c))
+//	'
+func TestGenerateSyncID(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		name string
+		tx   Transaction
+		want string
+	}{
+		{
+			name: "all fields set",
+			tx: Transaction{
+				Date: "2026-01-15", Amount: 500.0, Currency: "CZK",
+				Sender: "Jan Novak", VS: "123", Message: "clenske 1/2026", BankID: "abc123",
+			},
+			want: "4ac26598b6f23965380690172156a438a7e97a97dcedf222e5afe1afbe2c1bc4",
+		},
+		{
+			name: "currency empty defaults to CZK",
+			tx: Transaction{
+				Date: "2026-01-15", Amount: 500.0, Currency: "",
+				Sender: "Jan Novak", VS: "123", Message: "clenske 1/2026", BankID: "abc123",
+			},
+			want: "4ac26598b6f23965380690172156a438a7e97a97dcedf222e5afe1afbe2c1bc4",
+		},
+		{
+			name: "mixed-case fields lowercased before hashing",
+			tx: Transaction{
+				Date: "2026-02-10", Amount: 1234.56, Currency: "CZK",
+				Sender: "ABC SRO", VS: "", Message: "FAKTURA 42", BankID: "xyz",
+			},
+			want: "d40fa224d4fa572ffcd58e308e5c6508c4d5ca087b24ef6ff9284528fc128250",
+		},
+		{
+			name: "negative amount",
+			tx: Transaction{
+				Date: "2026-03-01", Amount: -500.0, Currency: "CZK",
+				Sender: "refund", VS: "", Message: "", BankID: "",
+			},
+			want: "0c630a407160367c396a2beec08efb94c319b4d84a8b90cc2be89e6ea10c391f",
+		},
+		{
+			name: "zero amount",
+			tx: Transaction{
+				Date: "2026-04-01", Amount: 0.0, Currency: "CZK",
+				Sender: "", VS: "", Message: "", BankID: "",
+			},
+			want: "6a23ce53717cd539064d550d2c2ec5de2e9bf81016d16852820ca9b8e259331f",
+		},
+		{
+			// Python equivalent: {"date":"","amount":0.0,"currency":"CZK","sender":"","vs":"","message":"","bank_id":""}
+			// Note: Python generate_sync_id({}) hashes "" for missing amount, not "0.0".
+			name: "zero-value Transaction",
+			tx:   Transaction{},
+			want: "d33d7e391f5a43f0192bb5a34c0ec15715139125678ecef8e1324af7d943b21d",
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			got := GenerateSyncID(tc.tx)
+			if got != tc.want {
+				t.Errorf("GenerateSyncID(%+v) = %q, want %q", tc.tx, got, tc.want)
+			}
+		})
+	}
+}
+
+// All expected strings verified against the live Python implementation on 2026-05-06:
+//
+//	PYTHONPATH=scripts:. python -c '
+//	for v in [0.0, 500.0, -500.0, 0.1, 1234.56, 99999.99, 1500000.0, 1e16, 1e-5]:
+//	    print(repr(v), "->", repr(str(v)))
+//	'
+func TestFormatAmount(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		in   float64
+		want string
+	}{
+		{0.0, "0.0"},
+		{500.0, "500.0"},
+		{-500.0, "-500.0"},
+		{0.1, "0.1"},
+		{1234.56, "1234.56"},
+		{99999.99, "99999.99"},
+		{1500000.0, "1500000.0"},
+		{1e16, "1e+16"},
+		{1e-5, "1e-05"},
+	}
+
+	for _, tc := range cases {
+		got := formatAmount(tc.in)
+		if got != tc.want {
+			t.Errorf("formatAmount(%v) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}