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>

docs/plans/2026-10-08-1026-pre-feature-cleanup-tasks.md

@@ -136,7 +136,9 @@ before new feature work starts. Companion to the Go rewrite tracker,
   - `warmup_cache()` runs at import of `app.py`, so `make test` makes real
     Drive and Sheets read calls and needs the credentials file.
   - Moot if Python is retired.
-- [ ] **T18** Set the time zone in the Go container.
+- [ ] **T18** Set the time zone in the Go container. Partly addressed
+  2026-10-08: the binary now embeds the zone database and the scheduled sync
+  uses `SYNC_TZ`; the "current month" logic below is still open.
   - The image has no time zone configured, so `time.Now()` is UTC and the
     "current month" flips one to two hours late at month boundaries.
   - The Python image sets `Europe/Prague`.
@@ -161,6 +163,12 @@ Plan: [2026-10-08-1103-api-parity-vs-months-to-show.md](2026-10-08-1103-api-pari
   dropped leading zeros. New rows now carry a text marker; existing rows are
   left as they are and are read correctly either way.
 
+## Features added since the audit
+
+- 2026-10-08 — Scheduled bank sync in the Go server, deliberately Go-only.
+  Plan: [2026-10-08-1216-scheduled-bank-sync.md](2026-10-08-1216-scheduled-bank-sync.md).
+  It also added a lock so two syncs cannot overlap (relevant to T8).
+
 ## Notes & decisions
 
 (Add entries as you go. Format: `YYYY-MM-DD — short note`.)