feat(search): reject invalid dir parameter with 400

Previously any value other than ASC/DESC silently defaulted to
DESC with no feedback. Now returns 400 Bad Request.

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

backend/src/test/java/org/raddatz/familienarchiv/controller/DocumentControllerTest.java

@@ -84,6 +84,13 @@ class DocumentControllerTest {
                 .andExpect(status().isBadRequest());
     }
 
+    @Test
+    @WithMockUser
+    void search_withInvalidDir_returns400() throws Exception {
+        mockMvc.perform(get("/api/documents/search").param("dir", "INVALID"))
+                .andExpect(status().isBadRequest());
+    }
+
     @Test
     @WithMockUser
     void search_withInvalidSort_returns400() throws Exception {