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/cmd/fuj/main.go

@@ -1,20 +1,25 @@
 package main
 
 import (
+	"bytes"
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"fuj-management/go/internal/config"
 	"fuj-management/go/internal/io/fio"
 	"fuj-management/go/internal/io/sheets"
 	"fuj-management/go/internal/logging"
+	"fuj-management/go/internal/scheduler"
 	"fuj-management/go/internal/services/banksync"
 	"fuj-management/go/internal/services/membership"
 	"fuj-management/go/internal/web"
 	"io"
 	"log/slog"
 	"os"
+	"strings"
 	"time"
+	_ "time/tzdata" // the container image has no zone database; SYNC_TZ needs one
 )
 
 // Injected at build time via -ldflags "-X main.version=... -X main.commit=... -X main.buildDate=..."
@@ -90,8 +95,16 @@ func serverCmd(args []string) {
 	}
 	fioClients := buildFioClients(cfg)
 
-	actions := web.ActionHandlers{
-		BankSync: func(ctx context.Context, out io.Writer) error {
+	syncTimes, syncLoc, err := config.LoadSyncSchedule()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "fuj server: %v\n", err)
+		os.Exit(2)
+	}
+
+	// One runner serves both the /sync-bank page and the scheduler, so the two
+	// can never run at the same time.
+	runner := banksync.NewRunner(
+		func(ctx context.Context, out io.Writer) error {
 			from, to := banksync.DefaultWindow(time.Now())
 
 			fmt.Fprintln(out, "=== Sync Fio Transactions ===")
@@ -110,6 +123,22 @@ func serverCmd(args []string) {
 			fmt.Fprintf(out, "Inferred %d row(s).\n", n)
 			return nil
 		},
+	)
+
+	actions := web.ActionHandlers{
+		BankSync: func(ctx context.Context, out io.Writer) error {
+			return runner.Run(ctx, "manual", out)
+		},
+		SyncInfo: func() string { return syncInfo(runner, syncTimes, syncLoc, time.Now()) },
+	}
+
+	if len(syncTimes) == 0 {
+		logger.Info("scheduled bank sync disabled (SYNC_SCHEDULE=off)")
+	} else {
+		logger.Info("scheduled bank sync enabled", "times", fmt.Sprint(syncTimes), "tz", syncLoc.String())
+		go scheduler.Run(ctx, syncTimes, syncLoc, logger, func(ctx context.Context) {
+			scheduledSync(ctx, logger, runner, sources)
+		})
 	}
 
 	build := web.BuildInfo{Version: version, Commit: commit, BuildDate: buildDate}
@@ -120,6 +149,55 @@ func serverCmd(args []string) {
 	}
 }
 
+// scheduledSyncTimeout bounds one scheduled run (two bank fetches plus sheet writes).
+const scheduledSyncTimeout = 10 * time.Minute
+
+// scheduledSync is the scheduler's job: the same sync → infer → flush-cache
+// sequence as the /sync-bank page, with the outcome written to the log.
+func scheduledSync(ctx context.Context, logger *slog.Logger, runner *banksync.Runner, sources membership.Sources) {
+	ctx, cancel := context.WithTimeout(ctx, scheduledSyncTimeout)
+	defer cancel()
+
+	logger.Info("scheduled bank sync starting")
+	var buf bytes.Buffer
+	err := runner.Run(ctx, "scheduled", &buf)
+	if errors.Is(err, banksync.ErrBusy) {
+		logger.Warn("scheduled bank sync skipped: a sync is already running")
+		return
+	}
+	output := strings.Join(strings.Fields(buf.String()), " ")
+	if _, ferr := sources.FlushCache(); ferr != nil {
+		logger.Error("scheduled bank sync: cache flush failed", "err", ferr)
+	}
+	if err != nil {
+		logger.Error("scheduled bank sync failed", "err", err, "output", output)
+		return
+	}
+	logger.Info("scheduled bank sync finished", "output", output)
+}
+
+// syncInfo renders the footer line: the last run since the server started
+// and the next scheduled one.
+func syncInfo(runner *banksync.Runner, times []scheduler.TimeOfDay, loc *time.Location, now time.Time) string {
+	const layout = "2006-01-02 15:04"
+	var parts []string
+	if st, ok := runner.Last(); ok {
+		result := "ok"
+		if st.Err != nil {
+			result = "FAILED"
+		}
+		parts = append(parts, fmt.Sprintf("last sync %s (%s) %s", st.Finished.In(loc).Format(layout), st.Trigger, result))
+	} else {
+		parts = append(parts, "no sync since start")
+	}
+	if next := scheduler.Next(now, times, loc); !next.IsZero() {
+		parts = append(parts, "next "+next.Format(layout))
+	} else {
+		parts = append(parts, "scheduled sync off")
+	}
+	return strings.Join(parts, " | ")
+}
+
 func feesCmd(args []string) {
 	fs := flag.NewFlagSet("fees", flag.ExitOnError)
 	fs.Usage = func() {

go/cmd/fuj/main_test.go

@@ -0,0 +1,35 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fuj-management/go/internal/scheduler"
+	"fuj-management/go/internal/services/banksync"
+	"io"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSyncInfo(t *testing.T) {
+	now := time.Date(2026, 10, 8, 12, 0, 0, 0, time.UTC)
+	four := []scheduler.TimeOfDay{{Hour: 4}}
+
+	ok := banksync.NewRunner(func(context.Context, io.Writer) error { return nil })
+	if got := syncInfo(ok, four, time.UTC, now); got != "no sync since start | next 2026-10-09 04:00" {
+		t.Errorf("before any run: %q", got)
+	}
+	if err := ok.Run(context.Background(), "scheduled", io.Discard); err != nil {
+		t.Fatal(err)
+	}
+	got := syncInfo(ok, nil, time.UTC, now)
+	if !strings.HasPrefix(got, "last sync ") || !strings.HasSuffix(got, "(scheduled) ok | scheduled sync off") {
+		t.Errorf("after ok run, schedule off: %q", got)
+	}
+
+	bad := banksync.NewRunner(func(context.Context, io.Writer) error { return errors.New("boom") })
+	_ = bad.Run(context.Background(), "manual", io.Discard)
+	if got := syncInfo(bad, four, time.UTC, now); !strings.Contains(got, "(manual) FAILED | next 2026-10-09 04:00") {
+		t.Errorf("after failed run: %q", got)
+	}
+}