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
+) {}