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/server.go

@@ -26,6 +26,9 @@ type ActionHandlers struct {
 	// BankSync runs sync+infer and writes a human-readable log to out.
 	// nil disables the /sync-bank action (renders an error instead).
 	BankSync func(ctx context.Context, out io.Writer) error
+	// SyncInfo returns a one-line summary of the last and next bank sync for
+	// the page footer. nil or an empty result shows nothing.
+	SyncInfo func() string
 }
 
 // Run registers routes and starts the HTTP server on addr.