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/templates/sync.tmpl

@@ -9,7 +9,7 @@
   {{end}}
   <pre class="sync-log{{if not .Success}} sync-log--error{{end}}">{{.Output}}</pre>
 {{else}}
-  <p class="description">Fetches Fio transactions for the current year, infers payment details, and flushes the cache.</p>
+  <p class="description">Fetches Fio transactions for the current year and the last two months of the previous one, infers payment details, and flushes the cache.</p>
 {{end}}
 <p><a href="/sync-bank" class="btn">{{if .Output}}Run Again{{else}}Run Sync{{end}}</a></p>
 {{end}}