fix(document): add @Transactional to read methods that access lazy collections

- getDocumentById: add @Transactional(readOnly=true) — calls
  tagService.resolveEffectiveColors(doc.getTags()) which requires an open
  session after the LAZY switch
- getRecentActivity: add @Transactional(readOnly=true) — callers may access
  tags/receivers on the returned list; keeps session open for @BatchSize fetches
- updateDocumentTags: add @Transactional — write method was missing annotation

Also adds @JsonIgnoreProperties({"hibernateLazyInitializer","handler"}) to
Person and Tag to prevent Jackson serialization errors on uninitialized
lazy proxies.

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

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentService.java

@@ -447,6 +447,7 @@ public class DocumentService {
         return saved;
     }
 
+    @Transactional
     public Document updateDocumentTags(UUID docId, List<String> tagNames) {
         Document doc = documentRepository.findById(docId)
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.DOCUMENT_NOT_FOUND, "Document not found: " + docId));
@@ -636,6 +637,7 @@ public class DocumentService {
     }
 
     // 0. Zuletzt aktive Dokumente (sortiert nach updatedAt DESC)
+    @Transactional(readOnly = true)
     public List<Document> getRecentActivity(int size) {
         return documentRepository.findAll(
                 PageRequest.of(0, size, Sort.by(Sort.Direction.DESC, "updatedAt"))
@@ -843,6 +845,7 @@ public class DocumentService {
         documentRepository.save(doc);
     }
 
+    @Transactional(readOnly = true)
     public Document getDocumentById(UUID id) {
         Document doc = documentRepository.findById(id)
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.DOCUMENT_NOT_FOUND, "Document not found: " + id));

backend/src/main/java/org/raddatz/familienarchiv/person/Person.java

@@ -1,6 +1,7 @@
 package org.raddatz.familienarchiv.person;
 
 import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import io.swagger.v3.oas.annotations.media.Schema;
 import jakarta.persistence.*;
 import lombok.*;
@@ -9,6 +10,8 @@ import org.raddatz.familienarchiv.user.DisplayNameFormatter;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.UUID;
+
+@JsonIgnoreProperties({"hibernateLazyInitializer", "handler"})
 @Entity
 @Table(name = "persons")
 @Data

backend/src/main/java/org/raddatz/familienarchiv/tag/Tag.java

@@ -2,10 +2,12 @@ package org.raddatz.familienarchiv.tag;
 
 import java.util.UUID;
 
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import io.swagger.v3.oas.annotations.media.Schema;
 import jakarta.persistence.*;
 import lombok.*;
 
+@JsonIgnoreProperties({"hibernateLazyInitializer", "handler"})
 @Entity
 @Data
 @NoArgsConstructor