docs(document): document scope-check bypass on getSummaryById

Clarify in the Javadoc that getSummaryById intentionally skips scope checks
and tag-colour resolution. This is safe under the current single-tenant model
and is explicitly used by JourneyItemService.append() to validate that a linked
document exists before persisting a JourneyItem.

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

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

@@ -1007,9 +1007,10 @@ public class DocumentService {
     }
 
     /**
-     * Lean document lookup for embedding in JourneyItemView. Skips
-     * {@code tagService.resolveEffectiveColors} — ×N items per journey GET is wasted
-     * work that summary consumers never read. Called within a caller-provided transaction.
+     * Lightweight summary lookup for internal use (e.g. journey item append validation).
+     * Intentionally skips scope checks and tag-colour resolution — safe only
+     * under the current single-tenant model where all authenticated users share
+     * the same document scope. Called within a caller-provided transaction.
      */
     public Document getSummaryById(UUID id) {
         return documentRepository.findById(id)