feat(go): scheduled bank sync (Go only)

The Go server now runs sync -> infer -> flush-cache by itself at the
times in SYNC_SCHEDULE (comma-separated HH:MM, default 04:00, "off"
disables), read in SYNC_TZ (default Europe/Prague). Nothing runs at
startup; an invalid value stops the server with an error.

- internal/scheduler: time-of-day parsing, Next, and the run loop
- banksync.Runner: one lock shared by /sync-bank and the scheduler so
  runs never overlap, plus the status of the last run
- footer shows the last sync since start and the next scheduled run
- binary embeds time/tzdata because the Alpine image has no zone data

Deliberately not implemented in the legacy Python app, which is being
phased out; recorded in CLAUDE.md under "Deliberately Go-only features".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

go/internal/web/html_handler_test.go

@@ -314,6 +314,28 @@ func TestServeSync(t *testing.T) {
 	}
 }
 
+func TestFooterSyncInfo(t *testing.T) {
+	renderer, err := web.NewRenderer()
+	if err != nil {
+		t.Fatalf("NewRenderer: %v", err)
+	}
+	b := web.BuildInfo{Version: "v0", Commit: "abc1234", BuildDate: "2026-01-01"}
+	get := func(actions web.ActionHandlers) string {
+		h := web.NewHTMLHandler(renderer, b, fixtureHandler(t), actions)
+		w := httptest.NewRecorder()
+		h.ServeFlushCacheGET(w, httptest.NewRequest(http.MethodGet, "/flush-cache", nil))
+		return w.Body.String()
+	}
+
+	const info = "last sync 2026-10-08 04:00 (scheduled) ok"
+	if body := get(web.ActionHandlers{SyncInfo: func() string { return info }}); !strings.Contains(body, "| "+info) {
+		t.Errorf("footer missing sync info %q", info)
+	}
+	if body := get(web.ActionHandlers{}); strings.Contains(body, "last sync") {
+		t.Errorf("footer shows sync info although none was provided")
+	}
+}
+
 func TestPaymentQRMarkup(t *testing.T) {
 	renderer, err := web.NewRenderer()
 	if err != nil {