domain crate code quality cleanup

strip all comments, extract tests to tests/ dirs,
remove #[allow(clippy::...)], extract magic numbers to
constants, refactor schedule engine private methods to
use param structs, add Default impls, clippy.toml for
persistence constructors

crates/domain/src/ports/settings.rs

@@ -1,21 +1,12 @@
-//! Application settings port.
-//!
-//! Key-value admin configuration (e.g. `library_sync_interval_hours`).
-//! Small enough to keep as a single trait rather than CQRS split.
-
 use async_trait::async_trait;
 
 use crate::errors::DomainResult;
 
-/// Port for general admin settings persistence (`app_settings` table).
 #[async_trait]
 pub trait AppSettingsRepository: Send + Sync {
-    /// Get a setting value by key. Returns `None` if not set.
     async fn get(&self, key: &str) -> DomainResult<Option<String>>;
 
-    /// Set a setting value (upsert).
     async fn set(&self, key: &str, value: &str) -> DomainResult<()>;
 
-    /// Get all settings as (key, value) pairs.
     async fn get_all(&self) -> DomainResult<Vec<(String, String)>>;
 }