feat(go): M6.6 — /qr, /sync-bank, /flush-cache, /version pages

- GET /qr: Czech QR Platba PNG; ports Python qr_code() exactly
  (account validation, amount clamping, * stripping, SPD format)
- GET /sync-bank: Fio sync → infer → cache flush with captured log
- GET+POST /flush-cache: form + action, shows deleted count
- GET /version: JSON alias of /api/version (Python parity)
- FlushCache() added to membership.Sources; wired through api.Handler
- web.ActionHandlers{BankSync} closure-based dep injection for sync
- New dep: github.com/skip2/go-qrcode
- TestQRBuildSPD (9 cases), TestServeQR, TestServeFlushCache{GET,POST},
  TestServeSync, TestServeVersion added

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

go/internal/web/qr.go

@@ -0,0 +1,50 @@
+package web
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+	"unicode/utf8"
+
+	qrcode "github.com/skip2/go-qrcode"
+)
+
+var validAccount = regexp.MustCompile(`^[A-Z]{2}\d{2,34}$|^\d{1,16}/\d{4}$`)
+
+// BuildSPD builds a Czech QR Platba SPD string, matching Python's qr_code handler.
+// Invalid account falls back to defaultAccount.
+// Amount is clamped to [0, 10_000_000]; non-numeric input becomes "0.00".
+// Message is truncated to 60 runes and stripped of '*' characters.
+func BuildSPD(account, amount, message, defaultAccount string) string {
+	if !validAccount.MatchString(account) {
+		account = defaultAccount
+	}
+
+	var amtStr string
+	var f float64
+	if _, err := fmt.Sscanf(amount, "%f", &f); err != nil || f < 0 || f > 10_000_000 {
+		amtStr = "0.00"
+	} else {
+		amtStr = fmt.Sprintf("%.2f", f)
+	}
+
+	if utf8.RuneCountInString(message) > 60 {
+		runes := []rune(message)
+		message = string(runes[:60])
+	}
+	message = strings.ReplaceAll(message, "*", "")
+
+	var accStr string
+	if parts := strings.SplitN(account, "/", 2); len(parts) == 2 {
+		accStr = parts[0] + "*BC:" + parts[1]
+	} else {
+		accStr = account
+	}
+
+	return fmt.Sprintf("SPD*1.0*ACC:%s*AM:%s*CC:CZK*MSG:%s", accStr, amtStr, message)
+}
+
+// RenderQRCode encodes payload as a PNG QR code (256×256, error correction Medium).
+func RenderQRCode(payload string) ([]byte, error) {
+	return qrcode.Encode(payload, qrcode.Medium, 256)
+}