test(document): add query-count assertions for findAll + findById entity graphs

Adds Hibernate statistics to the test config and two new tests in
DocumentRepositoryTest:
- findAll_withSpecAndPageable asserts ≤5 statements for 10 documents
  (currently RED: EAGER @ManyToMany generates 31 secondary SELECTs)
- findById regression guard verifies collections load in ≤2 statements

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

backend/src/test/resources/application-test.yaml

@@ -13,6 +13,10 @@ spring:
     password: test
   mail:
     host: localhost
+  jpa:
+    properties:
+      hibernate:
+        generate_statistics: true
 
 # Disable OTel SDK entirely in tests — prevents auto-configuration from loading resource providers
 # (e.g. AzureAppServiceResourceProvider) that fail against the semconv version used here.