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/infrastructure/services/dedup_service.rs

@@ -111,7 +111,7 @@ impl DedupService {
     }
 
     /// Creates a stub instance for testing — never hits PG or the filesystem.
-    #[cfg(test)]
+    #[cfg(any(test, feature = "integration_tests"))]
     pub fn new_stub() -> Self {
         let stub_pool = Arc::new(
             sqlx::pool::PoolOptions::<sqlx::Postgres>::new()