feat(training): track and display CER per training run

After each training run, the Character Error Rate (CER = 1 - accuracy),
loss, accuracy, and epoch count are now stored on the OcrTrainingRun
record and shown in the training history table.

Also adds the missing POST /api/ocr/segtrain endpoint and the
triggerSegTraining service method so the segmentation training card
can actually trigger training.

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

backend/src/main/resources/db/migration/V32__add_training_metrics.sql

@@ -0,0 +1,5 @@
+ALTER TABLE ocr_training_runs
+    ADD COLUMN cer      DOUBLE PRECISION,
+    ADD COLUMN loss     DOUBLE PRECISION,
+    ADD COLUMN accuracy DOUBLE PRECISION,
+    ADD COLUMN epochs   INT;