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.go

@@ -21,17 +21,26 @@ func NewHTMLHandler(r *Renderer, b BuildInfo, ah *api.Handler, actions ActionHan
 	return &HTMLHandler{renderer: r, build: b, apiHandler: ah, actions: actions}
 }
 
+// page builds the data shared by every page (nav highlight, build info, sync summary).
+func (h *HTMLHandler) page(active string) PageData {
+	pd := PageData{Active: active, Build: h.build}
+	if h.actions.SyncInfo != nil {
+		pd.SyncInfo = h.actions.SyncInfo()
+	}
+	return pd
+}
+
 func (h *HTMLHandler) ServeAdults(w http.ResponseWriter, r *http.Request) {
 	data, err := h.apiHandler.AssembleAdults(r.Context())
 	if err != nil {
 		h.renderer.Render(w, "adults", AdultsPageData{
-			PageData: PageData{Active: "adults", Build: h.build},
+			PageData: h.page("adults"),
 			Error:    err.Error(),
 		})
 		return
 	}
 	h.renderer.Render(w, "adults", AdultsPageData{
-		PageData: PageData{Active: "adults", Build: h.build},
+		PageData: h.page("adults"),
 		Data:     data,
 	})
 }
@@ -40,13 +49,13 @@ func (h *HTMLHandler) ServeJuniors(w http.ResponseWriter, r *http.Request) {
 	data, err := h.apiHandler.AssembleJuniors(r.Context())
 	if err != nil {
 		h.renderer.Render(w, "juniors", JuniorsPageData{
-			PageData: PageData{Active: "juniors", Build: h.build},
+			PageData: h.page("juniors"),
 			Error:    err.Error(),
 		})
 		return
 	}
 	h.renderer.Render(w, "juniors", JuniorsPageData{
-		PageData: PageData{Active: "juniors", Build: h.build},
+		PageData: h.page("juniors"),
 		Data:     data,
 	})
 }
@@ -55,20 +64,20 @@ func (h *HTMLHandler) ServePayments(w http.ResponseWriter, r *http.Request) {
 	data, err := h.apiHandler.AssemblePayments(r.Context())
 	if err != nil {
 		h.renderer.Render(w, "payments", PaymentsPageData{
-			PageData: PageData{Active: "payments", Build: h.build},
+			PageData: h.page("payments"),
 			Error:    err.Error(),
 		})
 		return
 	}
 	h.renderer.Render(w, "payments", PaymentsPageData{
-		PageData: PageData{Active: "payments", Build: h.build},
+		PageData: h.page("payments"),
 		Data:     data,
 	})
 }
 
 // ServeSync handles GET /sync-bank: runs sync+infer+flush then renders the result.
 func (h *HTMLHandler) ServeSync(w http.ResponseWriter, r *http.Request) {
-	pd := PageData{Active: "sync", Build: h.build}
+	pd := h.page("sync")
 
 	if h.actions.BankSync == nil {
 		h.renderer.Render(w, "sync", SyncPageData{
@@ -96,7 +105,7 @@ func (h *HTMLHandler) ServeSync(w http.ResponseWriter, r *http.Request) {
 	}
 
 	h.renderer.Render(w, "sync", SyncPageData{
-		PageData: pd,
+		PageData: h.page("sync"), // rebuilt so the footer reflects this run
 		Output:   buf.String(),
 		Success:  success,
 	})
@@ -105,7 +114,7 @@ func (h *HTMLHandler) ServeSync(w http.ResponseWriter, r *http.Request) {
 // 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},
+		PageData: h.page("flush"),
 	})
 }
 
@@ -113,7 +122,7 @@ func (h *HTMLHandler) ServeFlushCacheGET(w http.ResponseWriter, r *http.Request)
 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},
+		PageData: h.page("flush"),
 		Flushed:  true,
 		Deleted:  n,
 	})