refactor: move domain inline tests to separate files under tests/

Match the application crate convention: each source file references its
tests via #[cfg(test)] #[path = "tests/filename.rs"] mod tests; with
the test code in a sibling tests/ directory.

- events.rs       -> tests/events.rs
- value_objects.rs -> tests/value_objects.rs
- models/mod.rs   -> models/tests/mod.rs  (renamed from tests.rs)
- models/person.rs -> models/tests/person.rs
- models/goal.rs   -> models/tests/goal.rs
- models/watch_event.rs -> models/tests/watch_event.rs
- services/review_history.rs -> services/tests/review_history.rs

crates/domain/src/models/mod.rs

@@ -92,5 +92,5 @@ pub enum ExportFormat {
 }
 
 #[cfg(test)]
-#[path = "tests.rs"]
+#[path = "tests/mod.rs"]
 mod tests;