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/users/update_profile_fields.rs

@@ -0,0 +1,21 @@
+use domain::{errors::DomainError, events::DomainEvent, value_objects::UserId};
+
+use crate::{context::AppContext, users::commands::UpdateProfileFieldsCommand};
+
+pub async fn execute(ctx: &AppContext, cmd: UpdateProfileFieldsCommand) -> Result<(), DomainError> {
+    if cmd.fields.len() > 4 {
+        return Err(DomainError::ValidationError(
+            "Maximum 4 profile fields allowed".into(),
+        ));
+    }
+    let user_id = UserId::from_uuid(cmd.user_id);
+    ctx.repos
+        .profile_fields
+        .set_fields(&user_id, cmd.fields)
+        .await?;
+    ctx.services
+        .event_publisher
+        .publish(&DomainEvent::UserUpdated { user_id })
+        .await?;
+    Ok(())
+}