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).

go/internal/config/config_test.go

@@ -0,0 +1,45 @@
+package config
+
+import (
+	"testing"
+)
+
+func TestLoadSyncSchedule(t *testing.T) {
+	cases := []struct {
+		name, schedule, tz string
+		wantTimes          int
+		wantZone           string
+		wantErr            bool
+	}{
+		{"defaults", "", "", 1, "Europe/Prague", false},
+		{"two times", "06:00,18:00", "", 2, "Europe/Prague", false},
+		{"off", "off", "", 0, "Europe/Prague", false},
+		{"other zone", "04:00", "UTC", 1, "UTC", false},
+		{"bad time", "25:00", "", 0, "", true},
+		{"bad zone", "04:00", "Mars/Olympus", 0, "", true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("SYNC_SCHEDULE", tc.schedule)
+			t.Setenv("SYNC_TZ", tc.tz)
+			times, loc, err := LoadSyncSchedule()
+			if (err != nil) != tc.wantErr {
+				t.Fatalf("err = %v, wantErr %v", err, tc.wantErr)
+			}
+			if tc.wantErr {
+				return
+			}
+			if len(times) != tc.wantTimes {
+				t.Errorf("times = %v, want %d entries", times, tc.wantTimes)
+			}
+			if loc.String() != tc.wantZone {
+				t.Errorf("zone = %s, want %s", loc, tc.wantZone)
+			}
+		})
+	}
+	t.Setenv("SYNC_SCHEDULE", "")
+	t.Setenv("SYNC_TZ", "")
+	if times, _, _ := LoadSyncSchedule(); len(times) != 1 || times[0].String() != DefaultSyncSchedule {
+		t.Errorf("default schedule = %v, want [%s]", times, DefaultSyncSchedule)
+	}
+}