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/web/render.go

@@ -10,8 +10,9 @@ import (
 
 // PageData is the view model passed to every HTML template.
 type PageData struct {
-	Active string
-	Build  BuildInfo
+	Active   string
+	Build    BuildInfo
+	SyncInfo string // last/next bank sync summary shown in the footer; empty hides it
 }
 
 // AdultsPageData is the view model for the /adults HTML page.