feat(person): add findTopByDocumentCount endpoint for reader dashboard

PersonController GET /api/persons?sort=documentCount&size=N returns the top N
persons by combined sender+receiver document count for the reader dashboard.

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

backend/src/test/java/org/raddatz/familienarchiv/person/PersonControllerTest.java

@@ -81,6 +81,17 @@ class PersonControllerTest {
                 .andExpect(jsonPath("$[0].firstName").value("Hans"));
     }
 
+    @Test
+    @WithMockUser(authorities = "READ_ALL")
+    void getPersons_delegatesTopByDocumentCount_whenSortAndSizeGiven() throws Exception {
+        PersonSummaryDTO top = mockPersonSummary("Käthe", "Raddatz");
+        when(personService.findTopByDocumentCount(4)).thenReturn(List.of(top));
+
+        mockMvc.perform(get("/api/persons").param("sort", "documentCount").param("size", "4"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$[0].firstName").value("Käthe"));
+    }
+
     private PersonSummaryDTO mockPersonSummary(String firstName, String lastName) {
         return new PersonSummaryDTO() {
             public java.util.UUID getId() { return UUID.randomUUID(); }