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/register_and_login.rs

@@ -0,0 +1,32 @@
+use domain::errors::DomainError;
+
+use crate::{
+    auth::commands::RegisterAndLoginCommand,
+    auth::{login, register},
+    context::AppContext,
+};
+
+pub async fn execute(
+    ctx: &AppContext,
+    cmd: RegisterAndLoginCommand,
+) -> Result<login::LoginResult, DomainError> {
+    register::execute(
+        ctx,
+        crate::auth::commands::RegisterCommand {
+            email: cmd.email.clone(),
+            username: cmd.username,
+            password: cmd.password.clone(),
+            role: domain::models::UserRole::Standard,
+        },
+    )
+    .await?;
+
+    login::execute(
+        ctx,
+        crate::auth::queries::LoginQuery {
+            email: cmd.email,
+            password: cmd.password,
+        },
+    )
+    .await
+}