test: move application tests to tests/

test_support/test_helpers -> tests/support; watched_at unified w/ domain test_watched_at (2024-03-15).
Kept inline: users::diary_filter tests (feed_sort_to_direction_all_variants, paged_user_filter_builds_correctly) - pub(super) helpers in private module.
Dropped stale never-compiled users/tests/get_users.rs (old execute signature).

crates/application/tests/users/get_federated_profile_stats_test.rs

@@ -0,0 +1,37 @@
+use crate::support::TestContextBuilder;
+use application::users::deps::GetFederatedProfileStatsDeps;
+use application::users::get_federated_profile_stats;
+use application::users::queries::{GetUserProfileQuery, ProfileView};
+
+#[tokio::test]
+async fn succeeds_for_a_user_id_with_no_local_row() {
+    let b = TestContextBuilder::new();
+    let deps = GetFederatedProfileStatsDeps {
+        stats: b.stats_repo.clone(),
+        diary: b.diary_repo.clone(),
+        social_query: b.social_query_unified.clone(),
+    };
+
+    let synthetic_user_id = uuid::Uuid::new_v4();
+
+    let result = get_federated_profile_stats::execute(
+        &deps,
+        GetUserProfileQuery {
+            user_id: synthetic_user_id,
+            view: ProfileView::Recent,
+            limit: None,
+            offset: None,
+            sort_by: domain::models::FeedSortBy::Date,
+            search: None,
+            is_own_profile: false,
+            include_remote: true,
+        },
+    )
+    .await;
+
+    assert!(
+        result.is_ok(),
+        "execute must not hard-fail for a user_id absent from the local `users` table \
+         (the federated-profile call shape) — got Err"
+    );
+}