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_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)
+	}
+}