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/scheduler/scheduler.go

@@ -0,0 +1,95 @@
+// Package scheduler runs a job at fixed times of day.
+package scheduler
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"sort"
+	"strings"
+	"time"
+)
+
+// TimeOfDay is a wall-clock time without a date.
+type TimeOfDay struct {
+	Hour, Minute int
+}
+
+func (t TimeOfDay) String() string { return fmt.Sprintf("%02d:%02d", t.Hour, t.Minute) }
+
+// ParseTimes parses a comma-separated list of HH:MM times, e.g. "04:00" or
+// "06:00,18:00". "off" yields an empty schedule. The result is sorted and
+// de-duplicated.
+func ParseTimes(s string) ([]TimeOfDay, error) {
+	s = strings.TrimSpace(s)
+	if strings.EqualFold(s, "off") {
+		return nil, nil
+	}
+	seen := map[TimeOfDay]bool{}
+	var times []TimeOfDay
+	for _, part := range strings.Split(s, ",") {
+		part = strings.TrimSpace(part)
+		parsed, err := time.Parse("15:04", part)
+		if err != nil {
+			return nil, fmt.Errorf("invalid time %q: want HH:MM, a comma-separated list of them, or \"off\"", part)
+		}
+		t := TimeOfDay{Hour: parsed.Hour(), Minute: parsed.Minute()}
+		if !seen[t] {
+			seen[t] = true
+			times = append(times, t)
+		}
+	}
+	sort.Slice(times, func(i, j int) bool {
+		if times[i].Hour != times[j].Hour {
+			return times[i].Hour < times[j].Hour
+		}
+		return times[i].Minute < times[j].Minute
+	})
+	return times, nil
+}
+
+// Next returns the earliest scheduled instant strictly after now, with the
+// times read in loc. It returns the zero time when times is empty.
+func Next(now time.Time, times []TimeOfDay, loc *time.Location) time.Time {
+	local := now.In(loc)
+	var best time.Time
+	for dayOffset := 0; dayOffset <= 1; dayOffset++ {
+		for _, t := range times {
+			c := time.Date(local.Year(), local.Month(), local.Day()+dayOffset, t.Hour, t.Minute, 0, 0, loc)
+			if c.After(now) && (best.IsZero() || c.Before(best)) {
+				best = c
+			}
+		}
+	}
+	return best
+}
+
+// pollInterval bounds each wait so the loop follows the wall clock across
+// clock adjustments and host suspend instead of relying on one long timer.
+const pollInterval = time.Minute
+
+// Run calls job at every scheduled time until ctx is cancelled. Nothing runs
+// at startup. Runs do not overlap: the next time is computed after job returns.
+func Run(ctx context.Context, times []TimeOfDay, loc *time.Location, logger *slog.Logger, job func(context.Context)) {
+	for {
+		next := Next(time.Now(), times, loc)
+		if next.IsZero() {
+			return
+		}
+		logger.Info("next scheduled run", "at", next.Format(time.RFC3339))
+		for {
+			wait := time.Until(next)
+			if wait <= 0 {
+				break
+			}
+			timer := time.NewTimer(min(wait, pollInterval))
+			select {
+			case <-ctx.Done():
+				timer.Stop()
+				return
+			case <-timer.C:
+			}
+		}
+		job(ctx)
+	}
+}