docs: add Reader glossary entry + clarifying comments on specs and query

- GLOSSARY.md: defines "Reader" as the permission-derived role
  (isReader = !canWrite && !canAnnotate) — addresses @Markus blocker
- GeschichteSpecifications.hasAuthor: comment explains null = no restriction
  (PUBLISHED path) — addresses @Markus suggestion
- PersonRepository.findTopByDocumentCount: comment explains alias-in-ORDER-BY
  is intentional PostgreSQL behaviour — addresses @Markus suggestion

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

backend/src/main/java/org/raddatz/familienarchiv/geschichte/GeschichteSpecifications.java

@@ -42,6 +42,7 @@ public final class GeschichteSpecifications {
         };
     }
 
+    // null authorId → no restriction (PUBLISHED path passes null; Spring Data skips null predicates)
     public static Specification<Geschichte> hasAuthor(UUID authorId) {
         return (root, query, cb) ->
                 authorId == null ? null : cb.equal(root.get("author").get("id"), authorId);

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

@@ -69,6 +69,8 @@ public interface PersonRepository extends JpaRepository<Person, UUID> {
             nativeQuery = true)
     List<PersonSummaryDTO> searchWithDocumentCount(@Param("query") String query);
 
+    // ORDER BY uses the computed alias "documentCount" — valid PostgreSQL (aliases allowed in ORDER BY,
+    // unlike WHERE/HAVING). This is intentional; it would silently fail on MySQL or H2.
     @Query(value = """
             SELECT p.id, p.title, p.first_name AS firstName, p.last_name AS lastName,
                    p.person_type AS personType,