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/auth/register_test.rs

@@ -0,0 +1,73 @@
+use crate::support::TestContextBuilder;
+use crate::support::register_command;
+use std::sync::Arc;
+
+use domain::ports::UserRepository;
+use domain::testing::InMemoryStore;
+use domain::value_objects::Email;
+
+use application::auth::{commands::RegisterCommand, deps::RegisterDeps, register};
+
+fn cmd(email: &str) -> RegisterCommand {
+    register_command(email, "alice")
+}
+
+#[tokio::test]
+async fn registering_creates_the_user() {
+    let users = InMemoryStore::new();
+    let b = TestContextBuilder::new().with_users(Arc::clone(&users) as _);
+    let deps = RegisterDeps {
+        user: b.user_repo.clone(),
+        password_hasher: b.password_hasher.clone(),
+        config: b.config.clone(),
+    };
+
+    register::execute(&deps, cmd("alice@example.com"))
+        .await
+        .unwrap();
+
+    let email = Email::new("alice@example.com".into()).unwrap();
+    let user = users.find_by_email(&email).await.unwrap().unwrap();
+    assert_eq!(user.email().value(), "alice@example.com");
+    assert!(user.password_hash().value().starts_with("hashed:"));
+}
+
+#[tokio::test]
+async fn registering_with_a_duplicate_email_fails() {
+    let users = InMemoryStore::new();
+    let b = TestContextBuilder::new().with_users(Arc::clone(&users) as _);
+    let deps = RegisterDeps {
+        user: b.user_repo.clone(),
+        password_hasher: b.password_hasher.clone(),
+        config: b.config.clone(),
+    };
+
+    register::execute(&deps, cmd("bob@example.com"))
+        .await
+        .unwrap();
+    let result = register::execute(&deps, cmd("bob@example.com")).await;
+    assert!(result.is_err(), "duplicate email should fail");
+}
+
+#[tokio::test]
+async fn registering_with_a_short_password_fails() {
+    let b = TestContextBuilder::new();
+    let deps = RegisterDeps {
+        user: b.user_repo.clone(),
+        password_hasher: b.password_hasher.clone(),
+        config: b.config.clone(),
+    };
+    let result = register::execute(
+        &deps,
+        RegisterCommand {
+            password: "short".to_string(),
+            ..register_command("x@y.com", "testuser")
+        },
+    )
+    .await;
+    let err = result.unwrap_err().to_string();
+    assert!(
+        err.contains("8 characters"),
+        "expected password length error, got: {err}"
+    );
+}