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/config/config.go

@@ -1,6 +1,8 @@
 package config
 
 import (
+	"fmt"
+	"fuj-management/go/internal/scheduler"
 	"os"
 	"strconv"
 	"strings"
@@ -92,6 +94,28 @@ func Load() Config {
 	}
 }
 
+// Scheduled bank sync defaults (Go server only; the legacy Python app has no scheduler).
+const (
+	DefaultSyncSchedule = "04:00"         // SYNC_SCHEDULE: comma-separated HH:MM times, or "off"
+	DefaultSyncTimezone = "Europe/Prague" // SYNC_TZ: IANA zone the schedule is read in
+)
+
+// LoadSyncSchedule reads SYNC_SCHEDULE and SYNC_TZ. An empty schedule means
+// the scheduled sync is disabled. Resolving the zone needs a zone database:
+// the server binary embeds one via time/tzdata.
+func LoadSyncSchedule() ([]scheduler.TimeOfDay, *time.Location, error) {
+	times, err := scheduler.ParseTimes(env("SYNC_SCHEDULE", DefaultSyncSchedule))
+	if err != nil {
+		return nil, nil, fmt.Errorf("SYNC_SCHEDULE: %w", err)
+	}
+	tz := env("SYNC_TZ", DefaultSyncTimezone)
+	loc, err := time.LoadLocation(tz)
+	if err != nil {
+		return nil, nil, fmt.Errorf("SYNC_TZ: unknown time zone %q", tz)
+	}
+	return times, loc, nil
+}
+
 // IBANAccountNum extracts the bare account number from a Czech IBAN.
 // "CZ8520100000002800359168" → "2800359168"
 // Structure: CZ(2 check)(4 bank code)(16 zero-padded account).