feat(transcription): PersonMention @Embeddable for sidecar entries

Value object held in TranscriptionBlock.mentionedPersons via @ElementCollection.
Carries the personId UUID (so renamed persons can be located) and the
displayName text (so block.text rewrites match exactly via "@" + name). Both
fields are non-null; displayName capped at 200 chars to match the V56 column
and bound the rename propagation cost.

Refs #362

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

backend/src/main/java/org/raddatz/familienarchiv/model/PersonMention.java

@@ -0,0 +1,30 @@
+package org.raddatz.familienarchiv.model;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+import jakarta.persistence.Column;
+import jakarta.persistence.Embeddable;
+import jakarta.validation.constraints.NotNull;
+import jakarta.validation.constraints.Size;
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+import java.util.UUID;
+
+@Embeddable
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+public class PersonMention {
+
+    @NotNull
+    @Column(name = "person_id", nullable = false)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private UUID personId;
+
+    @NotNull
+    @Size(max = 200)
+    @Column(name = "display_name", nullable = false, length = 200)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private String displayName;
+}