refactor: apply clippy let-chains and harden password validation

- Auto-fix 29 clippy warnings by converting nested if-lets to let-chains
- Harden password strength validator to reject restricted substrings
- Simplify rate_limiter state checks using is_none_or
- Improves idiomatic Rust style and overall security posture

src/identity/applications.rs

@@ -324,11 +324,9 @@ pub async fn update(
         .find_by_slug(slug)
         .await
         .map_err(AppError::Database)?
-    {
-        if other.entity_id != id || other.entity_type != "application" {
+        && (other.entity_id != id || other.entity_type != "application") {
             return Err(AppError::Conflict(format!("slug '{slug}' already exists")));
         }
-    }
 
     let redirect_json = redirect_uris.map(|v| serde_json::to_string(&v).unwrap_or_default());
     let scopes_json = scopes.map(|v| serde_json::to_string(&v).unwrap_or_default());