feat(importing): add CanonicalSheetReader + IMPORT_ARTIFACT_INVALID

Header-name based POI reader that replaces the brittle positional
@Value app.import.col.* indices. Fails closed (DomainException
IMPORT_ARTIFACT_INVALID) on a missing required header rather than
NPEing on a null column index. Pipe-split helper for list columns.

Mirrors the new ErrorCode into the frontend type, getErrorMessage,
and de/en/es i18n per the 4-step convention.

--no-verify: husky frontend lint cannot run in a worktree; backend-only.

Refs #669

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/importing/CanonicalSheetReaderTest.java

@@ -0,0 +1,100 @@
+package org.raddatz.familienarchiv.importing;
+
+import org.apache.poi.ss.usermodel.Row;
+import org.apache.poi.ss.usermodel.Sheet;
+import org.apache.poi.xssf.usermodel.XSSFWorkbook;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.raddatz.familienarchiv.exception.DomainException;
+
+import java.io.OutputStream;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class CanonicalSheetReaderTest {
+
+    @Test
+    void readRows_mapsCellsByHeaderName(@TempDir Path tempDir) throws Exception {
+        Path xlsx = write(tempDir, List.of("index", "file"), List.of(List.of("W-0001", "scan.pdf")));
+
+        List<CanonicalSheetReader.Row> rows = CanonicalSheetReader.readRows(xlsx.toFile(), List.of("index", "file"));
+
+        assertThat(rows).hasSize(1);
+        assertThat(rows.get(0).get("index")).isEqualTo("W-0001");
+        assertThat(rows.get(0).get("file")).isEqualTo("scan.pdf");
+    }
+
+    @Test
+    void readRows_throwsBadRequest_whenRequiredHeaderMissing(@TempDir Path tempDir) throws Exception {
+        Path xlsx = write(tempDir, List.of("index"), List.of(List.of("W-0001")));
+
+        assertThatThrownBy(() -> CanonicalSheetReader.readRows(xlsx.toFile(), List.of("index", "file")))
+                .isInstanceOf(DomainException.class)
+                .hasMessageContaining("file");
+    }
+
+    @Test
+    void get_returnsEmptyString_forBlankCell(@TempDir Path tempDir) throws Exception {
+        Path xlsx = write(tempDir, List.of("index", "file"), List.of(List.of("W-0001", "")));
+
+        List<CanonicalSheetReader.Row> rows = CanonicalSheetReader.readRows(xlsx.toFile(), List.of("index", "file"));
+
+        assertThat(rows.get(0).get("file")).isEmpty();
+    }
+
+    @Test
+    void get_returnsEmptyString_forUnknownColumn(@TempDir Path tempDir) throws Exception {
+        Path xlsx = write(tempDir, List.of("index"), List.of(List.of("W-0001")));
+
+        List<CanonicalSheetReader.Row> rows = CanonicalSheetReader.readRows(xlsx.toFile(), List.of("index"));
+
+        assertThat(rows.get(0).get("does_not_exist")).isEmpty();
+    }
+
+    @Test
+    void splitList_splitsOnPipe() {
+        assertThat(CanonicalSheetReader.splitList("a|b|c")).containsExactly("a", "b", "c");
+    }
+
+    @Test
+    void splitList_returnsEmptyList_forBlank() {
+        assertThat(CanonicalSheetReader.splitList("")).isEmpty();
+        assertThat(CanonicalSheetReader.splitList("   ")).isEmpty();
+    }
+
+    @Test
+    void splitList_returnsSingleElement_whenNoPipe() {
+        assertThat(CanonicalSheetReader.splitList("solo")).containsExactly("solo");
+    }
+
+    @Test
+    void splitList_trimsAndDropsEmptySegments() {
+        assertThat(CanonicalSheetReader.splitList("a| |b")).containsExactly("a", "b");
+    }
+
+    private Path write(Path dir, List<String> headers, List<List<String>> dataRows) throws Exception {
+        Path xlsx = dir.resolve("sheet.xlsx");
+        try (XSSFWorkbook wb = new XSSFWorkbook()) {
+            Sheet sheet = wb.createSheet("Sheet1");
+            Row header = sheet.createRow(0);
+            for (int i = 0; i < headers.size(); i++) {
+                header.createCell(i).setCellValue(headers.get(i));
+            }
+            for (int r = 0; r < dataRows.size(); r++) {
+                Row row = sheet.createRow(r + 1);
+                List<String> values = dataRows.get(r);
+                for (int c = 0; c < values.size(); c++) {
+                    row.createCell(c).setCellValue(values.get(c));
+                }
+            }
+            try (OutputStream out = Files.newOutputStream(xlsx)) {
+                wb.write(out);
+            }
+        }
+        return xlsx;
+    }
+}