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/share_service.rs

@@ -430,9 +430,11 @@ impl ShareUseCase for ShareService {
     }
 }
 
-#[cfg(integration_tests)]
+#[cfg(feature = "integration_tests")]
+#[allow(dead_code)]
 mod tests {
     use super::*;
+    #[allow(unused_imports)]
     use crate::application::dtos::share_dto::SharePermissionsDto;
     use crate::application::ports::auth_ports::PasswordHasherPort;
     use crate::application::ports::share_ports::ShareStoragePort;