refactor(fts): address PR #488 review concerns

- Extract isPureTextRelevance() private static method to replace the
  7-clause inline boolean in searchDocuments
- Guard long→int cast in relevanceSortedPageFromSql to prevent silent
  overflow at page ≥43M (CWE-190)
- resolvePersonName now uses the typed API client (createApiClient)
  instead of raw fetch, aligning with project conventions
- Update DocumentServiceTest stubs to match new FTS path (findFtsPageRaw
  + findAllById instead of findAllMatchingIdsByFts)
- Rewrite page.server.spec.ts person-name tests to mock via path-based
  API dispatch, matching the new api.GET call site

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

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

@@ -647,10 +647,7 @@ public class DocumentService {
         boolean hasText = StringUtils.hasText(text);
 
         // Pure-text RELEVANCE: push pagination into SQL — skip findAllMatchingIdsByFts entirely (ADR-008).
-        boolean pureTextRelevance = hasText && (sort == null || sort == DocumentSort.RELEVANCE)
-                && from == null && to == null && sender == null && receiver == null
-                && (tags == null || tags.isEmpty()) && (tagQ == null || tagQ.isBlank()) && status == null;
-        if (pureTextRelevance) {
+        if (isPureTextRelevance(hasText, sort, from, to, sender, receiver, tags, tagQ, status)) {
             return relevanceSortedPageFromSql(text, pageable);
         }
 
@@ -695,12 +692,22 @@ public class DocumentService {
         return buildResultPaged(page.getContent(), text, pageable, page.getTotalElements());
     }
 
+    private static boolean isPureTextRelevance(boolean hasText, DocumentSort sort,
+            LocalDate from, LocalDate to, UUID sender, UUID receiver,
+            List<String> tags, String tagQ, DocumentStatus status) {
+        return hasText && (sort == null || sort == DocumentSort.RELEVANCE)
+                && from == null && to == null && sender == null && receiver == null
+                && (tags == null || tags.isEmpty()) && (tagQ == null || tagQ.isBlank()) && status == null;
+    }
+
     /**
      * Pure-text RELEVANCE path — pagination and ts_rank ordering pushed into SQL.
      * Called when no non-text filters are active (ADR-008).
      */
     private DocumentSearchResult relevanceSortedPageFromSql(String text, Pageable pageable) {
-        int offset = (int) pageable.getOffset();
+        long rawOffset = pageable.getOffset();
+        if (rawOffset > Integer.MAX_VALUE) return DocumentSearchResult.of(List.of());
+        int offset = (int) rawOffset;
         int limit = pageable.getPageSize();
         FtsPage ftsPage = toFtsPage(documentRepository.findFtsPageRaw(text, offset, limit));
         if (ftsPage.hits().isEmpty()) return DocumentSearchResult.of(List.of());