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>

legacy/app.py

@@ -121,6 +121,8 @@ def flush_cache_endpoint():
     deleted = flush_cache()
     return render_template("flush-cache.html", flushed=True, deleted=deleted)
 
+# Scheduled (daily) bank sync exists only in the Go server, on purpose: this
+# app is legacy and being phased out. Here the sync runs only on request.
 @app.route("/sync-bank")
 def sync_bank():
     import contextlib