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/html_handler.go

@@ -1,8 +1,11 @@
 package web
 
 import (
+	"bytes"
+	"fmt"
 	"fuj-management/go/internal/web/api"
 	"net/http"
+	"runtime/debug"
 )
 
 // HTMLHandler serves the Go-native HTML frontend.
@@ -10,11 +13,12 @@ type HTMLHandler struct {
 	renderer   *Renderer
 	build      BuildInfo
 	apiHandler *api.Handler
+	actions    ActionHandlers
 }
 
 // NewHTMLHandler constructs an HTMLHandler.
-func NewHTMLHandler(r *Renderer, b BuildInfo, ah *api.Handler) *HTMLHandler {
-	return &HTMLHandler{renderer: r, build: b, apiHandler: ah}
+func NewHTMLHandler(r *Renderer, b BuildInfo, ah *api.Handler, actions ActionHandlers) *HTMLHandler {
+	return &HTMLHandler{renderer: r, build: b, apiHandler: ah, actions: actions}
 }
 
 func (h *HTMLHandler) ServeAdults(w http.ResponseWriter, r *http.Request) {
@@ -62,10 +66,78 @@ func (h *HTMLHandler) ServePayments(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// ServeSync handles GET /sync-bank: runs sync+infer+flush then renders the result.
 func (h *HTMLHandler) ServeSync(w http.ResponseWriter, r *http.Request) {
-	h.renderer.Render(w, "sync", PageData{Active: "sync", Build: h.build})
+	pd := PageData{Active: "sync", Build: h.build}
+
+	if h.actions.BankSync == nil {
+		h.renderer.Render(w, "sync", SyncPageData{
+			PageData: pd,
+			Output:   "Bank sync is not configured.",
+			Success:  false,
+		})
+		return
+	}
+
+	var buf bytes.Buffer
+	success := true
+	if err := h.actions.BankSync(r.Context(), &buf); err != nil {
+		fmt.Fprintf(&buf, "\nError: %s\n\nStack trace:\n%s", err.Error(), debug.Stack())
+		success = false
+	}
+
+	fmt.Fprintln(&buf, "\n=== Flush Cache ===")
+	n, err := h.apiHandler.FlushCache()
+	if err != nil {
+		fmt.Fprintf(&buf, "flush error: %s\n", err.Error())
+		success = false
+	} else {
+		fmt.Fprintf(&buf, "%d cache file(s) deleted.\n", n)
+	}
+
+	h.renderer.Render(w, "sync", SyncPageData{
+		PageData: pd,
+		Output:   buf.String(),
+		Success:  success,
+	})
 }
 
-func (h *HTMLHandler) ServeFlushCache(w http.ResponseWriter, r *http.Request) {
-	h.renderer.Render(w, "flush_cache", PageData{Active: "flush", Build: h.build})
+// ServeFlushCacheGET handles GET /flush-cache: renders the confirmation form.
+func (h *HTMLHandler) ServeFlushCacheGET(w http.ResponseWriter, r *http.Request) {
+	h.renderer.Render(w, "flush_cache", FlushPageData{
+		PageData: PageData{Active: "flush", Build: h.build},
+	})
+}
+
+// ServeFlushCachePOST handles POST /flush-cache: flushes and re-renders with count.
+func (h *HTMLHandler) ServeFlushCachePOST(w http.ResponseWriter, r *http.Request) {
+	n, _ := h.apiHandler.FlushCache()
+	h.renderer.Render(w, "flush_cache", FlushPageData{
+		PageData: PageData{Active: "flush", Build: h.build},
+		Flushed:  true,
+		Deleted:  n,
+	})
+}
+
+// ServeQR handles GET /qr: generates and returns a Czech QR Platba PNG.
+func (h *HTMLHandler) ServeQR(w http.ResponseWriter, r *http.Request) {
+	q := r.URL.Query()
+	account := q.Get("account")
+	amount := q.Get("amount")
+	message := q.Get("message")
+	if account == "" {
+		account = h.apiHandler.Config.BankAccount
+	}
+	if amount == "" {
+		amount = "0"
+	}
+
+	payload := BuildSPD(account, amount, message, h.apiHandler.Config.BankAccount)
+	png, err := RenderQRCode(payload)
+	if err != nil {
+		http.Error(w, "qr encode: "+err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "image/png")
+	_, _ = w.Write(png)
 }