fix(training): use ketos CLI subprocess instead of missing Python API

kraken.ketos has no .train or .segtrain attributes in Kraken 7 — both are
only exposed as CLI commands. Rewrites both training functions to invoke
`ketos train` / `ketos segtrain` via subprocess and parse the best
val_metric from checkpoint filenames.

Also fixes the OcrTrainingCard history so it only shows non-blla runs
(recognition model), matching SegmentationTrainingCard which already
filtered to blla-only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/OcrTrainingCard.svelte

@@ -88,5 +88,5 @@ async function startTraining() {
 	<h3 class="mt-6 mb-3 text-xs font-bold tracking-widest text-ink-3 uppercase">
 		{m.training_history_heading()}
 	</h3>
-	<TrainingHistory runs={trainingInfo?.runs ?? []} />
+	<TrainingHistory runs={(trainingInfo?.runs ?? []).filter((r) => r.modelName !== 'blla')} />
 </div>