fix: resolve all clippy warnings for CI (async_fn_in_trait, collapsible_if, type_complexity, dead_code)

- Allow async_fn_in_trait lint crate-wide (internal project, 413 warnings)
- Add integration_tests feature to Cargo.toml to fix unexpected cfg warnings
- Collapse nested if statements into single conditions (13 locations)
- Add type_complexity allows on pg repository functions (12 locations)
- Fix dead code warnings in test modules with allow attributes
- Fix E0599 by gating new_stub() for integration_tests feature
- Add result_unit_err and result_large_err allows where appropriate
- Apply rustfmt formatting

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

src/application/services/idor_protection_test.rs

@@ -131,17 +131,20 @@ impl FileReadPort for MockFileReadPort {
 }
 
 /// Minimal mock write port — only `move_file` and `rename_file` need real logic.
+#[allow(dead_code)]
 struct MockFileWritePort {
     files: Mutex<HashMap<String, File>>,
 }
 
 impl MockFileWritePort {
+    #[allow(dead_code)]
     fn new() -> Self {
         Self {
             files: Mutex::new(HashMap::new()),
         }
     }
 
+    #[allow(dead_code)]
     fn insert(&self, id: &str, name: &str) {
         let file = File::new(
             id.to_string(),