fix(training): switch to PAGE XML format for kurrent recognition training

Kraken 7 removed support for the legacy `path` format (image + .gt.txt
pairs) in VGSLRecognitionDataModule despite the CLI still advertising it.
Switching to PAGE XML (-f page) format which is the supported standard.

- Java export now writes .xml alongside .png (PAGE XML with TextLine,
  Baseline at 75% height, and Unicode transcription)
- XML special characters in transcription text are escaped (& < >)
- Python trainer globs *.xml and passes -f page to ketos train
- Regenerated frontend API types to include cer/loss/accuracy/epochs on
  OcrTrainingRun (were missing, causing empty CER column in history)
- Updated and extended TrainingDataExportServiceTest

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

backend/src/main/java/org/raddatz/familienarchiv/service/TrainingDataExportService.java

@@ -127,15 +127,47 @@ public class TrainingDataExportService {
 
     void writeTrainingPair(ZipOutputStream zip, UUID blockId, BufferedImage image, String text) throws IOException {
         String base = blockId.toString();
+        int w = image.getWidth();
+        int h = image.getHeight();
+        // Baseline at 75 % height — typical text baseline position in a cropped line image
+        int baselineY = (h * 3) / 4;
 
         // Write PNG
         zip.putNextEntry(new ZipEntry(base + ".png"));
         ImageIO.write(image, "PNG", zip);
         zip.closeEntry();
 
-        // Write ground-truth text
-        zip.putNextEntry(new ZipEntry(base + ".gt.txt"));
-        zip.write((text != null ? text : "").getBytes(StandardCharsets.UTF_8));
+        // Write PAGE XML (Kraken 7+ dropped the legacy "path" format)
+        String safeText = escapeXml(text != null ? text : "");
+        String xml = String.format(
+                "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n" +
+                "<PcGts xmlns=\"http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15\">\n" +
+                "  <Metadata><Creator>familienarchiv</Creator></Metadata>\n" +
+                "  <Page imageFilename=\"%s.png\" imageWidth=\"%d\" imageHeight=\"%d\">\n" +
+                "    <TextRegion id=\"r0\" type=\"paragraph\">\n" +
+                "      <Coords points=\"0,0 %d,0 %d,%d 0,%d\"/>\n" +
+                "      <TextLine id=\"l0\">\n" +
+                "        <Coords points=\"0,0 %d,0 %d,%d 0,%d\"/>\n" +
+                "        <Baseline points=\"0,%d %d,%d\"/>\n" +
+                "        <TextEquiv><Unicode>%s</Unicode></TextEquiv>\n" +
+                "      </TextLine>\n" +
+                "    </TextRegion>\n" +
+                "  </Page>\n" +
+                "</PcGts>\n",
+                base, w, h,
+                w - 1, w - 1, h - 1, h - 1,
+                w - 1, w - 1, h - 1, h - 1,
+                baselineY, w - 1, baselineY,
+                safeText);
+
+        zip.putNextEntry(new ZipEntry(base + ".xml"));
+        zip.write(xml.getBytes(StandardCharsets.UTF_8));
         zip.closeEntry();
     }
+
+    private static String escapeXml(String text) {
+        return text.replace("&", "&amp;")
+                   .replace("<", "&lt;")
+                   .replace(">", "&gt;");
+    }
 }