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/events.rs

@@ -135,21 +135,5 @@ impl EventEnvelope {
 }
 
 #[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::value_objects::UserId;
-
-    #[test]
-    fn follow_accepted_matches() {
-        let uid = UserId::from_uuid(uuid::Uuid::new_v4());
-        let event = DomainEvent::FollowAccepted {
-            local_user_id: uid.clone(),
-            remote_actor_url: "https://remote.example/users/alice".to_string(),
-            outbox_url: "https://remote.example/users/alice/outbox".to_string(),
-        };
-        let DomainEvent::FollowAccepted { outbox_url, .. } = event else {
-            panic!("wrong variant");
-        };
-        assert_eq!(outbox_url, "https://remote.example/users/alice/outbox");
-    }
-}
+#[path = "tests/events.rs"]
+mod tests;