refactor: group use cases into DDD bounded contexts

Flat use_cases/ (44 files) + monolithic commands.rs/queries.rs
split into diary/, movies/, watchlist/, import/, auth/, users/,
integrations/, search/, person/, federation/ — each with own
commands.rs, queries.rs, and use case modules.

Inline tests extracted to sibling tests/ dirs.

crates/application/src/auth/tests/register.rs

@@ -0,0 +1,48 @@
+use std::sync::Arc;
+
+use domain::models::UserRole;
+use domain::ports::UserRepository;
+use domain::testing::InMemoryUserRepository;
+use domain::value_objects::Email;
+
+use crate::{auth::commands::RegisterCommand, auth::register, test_helpers::TestContextBuilder};
+
+fn cmd(email: &str) -> RegisterCommand {
+    RegisterCommand {
+        email: email.to_string(),
+        username: "alice".to_string(),
+        password: "password123".to_string(),
+        role: UserRole::Standard,
+    }
+}
+
+#[tokio::test]
+async fn test_register_creates_user() {
+    let users = InMemoryUserRepository::new();
+    let ctx = TestContextBuilder::new()
+        .with_users(Arc::clone(&users) as _)
+        .build();
+
+    register::execute(&ctx, 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 test_register_duplicate_email_fails() {
+    let users = InMemoryUserRepository::new();
+    let ctx = TestContextBuilder::new()
+        .with_users(Arc::clone(&users) as _)
+        .build();
+
+    register::execute(&ctx, cmd("bob@example.com"))
+        .await
+        .unwrap();
+    let result = register::execute(&ctx, cmd("bob@example.com")).await;
+    assert!(result.is_err(), "duplicate email should fail");
+}