feat(geschichte): add entity, status enum, and V58 schema migration

Geschichte holds family memory stories (issue #381). Body is unbounded TEXT
(Tiptap HTML, no length limit). Two join tables link a story to historical
Persons and Documents. A partial index speeds the public index query
(status='PUBLISHED' ORDER BY published_at DESC) and reverse-lookup indexes
support the ?personId and ?documentId filters.

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

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

@@ -0,0 +1,69 @@
+package org.raddatz.familienarchiv.model;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+import jakarta.persistence.*;
+import lombok.*;
+import org.hibernate.annotations.CreationTimestamp;
+import org.hibernate.annotations.UpdateTimestamp;
+
+import java.time.LocalDateTime;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.UUID;
+
+@Entity
+@Table(name = "geschichten")
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+@Builder
+public class Geschichte {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.UUID)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private UUID id;
+
+    @Column(nullable = false)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private String title;
+
+    @Column(columnDefinition = "TEXT")
+    private String body;
+
+    @Enumerated(EnumType.STRING)
+    @Column(nullable = false)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    @Builder.Default
+    private GeschichteStatus status = GeschichteStatus.DRAFT;
+
+    @ManyToOne
+    @JoinColumn(name = "author_id")
+    private AppUser author;
+
+    @ManyToMany(fetch = FetchType.EAGER)
+    @JoinTable(name = "geschichten_persons",
+            joinColumns = @JoinColumn(name = "geschichte_id"),
+            inverseJoinColumns = @JoinColumn(name = "person_id"))
+    @Builder.Default
+    private Set<Person> persons = new HashSet<>();
+
+    @ManyToMany(fetch = FetchType.EAGER)
+    @JoinTable(name = "geschichten_documents",
+            joinColumns = @JoinColumn(name = "geschichte_id"),
+            inverseJoinColumns = @JoinColumn(name = "document_id"))
+    @Builder.Default
+    private Set<Document> documents = new HashSet<>();
+
+    @CreationTimestamp
+    @Column(updatable = false)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private LocalDateTime createdAt;
+
+    @UpdateTimestamp
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
+    private LocalDateTime updatedAt;
+
+    @Column(name = "published_at")
+    private LocalDateTime publishedAt;
+}

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

@@ -0,0 +1,6 @@
+package org.raddatz.familienarchiv.model;
+
+public enum GeschichteStatus {
+    DRAFT,
+    PUBLISHED
+}