feat(search): add MatchOffset record for character-level highlight positions

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

backend/src/main/java/org/raddatz/familienarchiv/dto/MatchOffset.java

@@ -0,0 +1,14 @@
+package org.raddatz.familienarchiv.dto;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+/**
+ * Character-level offset of a highlighted term within a text field.
+ * Offsets are Java {@code String} character positions (UTF-16 code units),
+ * which are identical to JavaScript string positions — consistent end-to-end
+ * for all German BMP characters (ä, ö, ü, ß, etc.).
+ */
+public record MatchOffset(
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) int start,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) int length
+) {}

backend/src/test/java/org/raddatz/familienarchiv/dto/MatchOffsetTest.java

@@ -0,0 +1,22 @@
+package org.raddatz.familienarchiv.dto;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class MatchOffsetTest {
+
+    @Test
+    void should_hold_start_and_length() {
+        MatchOffset offset = new MatchOffset(6, 5);
+
+        assertThat(offset.start()).isEqualTo(6);
+        assertThat(offset.length()).isEqualTo(5);
+    }
+
+    @Test
+    void should_implement_value_equality() {
+        assertThat(new MatchOffset(0, 3)).isEqualTo(new MatchOffset(0, 3));
+        assertThat(new MatchOffset(0, 3)).isNotEqualTo(new MatchOffset(0, 4));
+    }
+}