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/watchlist/add.rs

@@ -0,0 +1,69 @@
+use domain::{
+    errors::DomainError,
+    events::DomainEvent,
+    models::WatchlistEntry,
+    value_objects::{MovieId, UserId},
+};
+
+use crate::{
+    context::AppContext,
+    diary::movie_resolver::{MovieResolver, MovieResolverDeps},
+    watchlist::commands::AddToWatchlistCommand,
+};
+
+pub async fn execute(ctx: &AppContext, cmd: AddToWatchlistCommand) -> Result<(), DomainError> {
+    let user_id = UserId::from_uuid(cmd.user_id);
+
+    let movie = if let Some(id) = cmd.input.movie_id {
+        let movie_id = MovieId::from_uuid(id);
+        ctx.repos
+            .movie
+            .get_movie_by_id(&movie_id)
+            .await?
+            .ok_or_else(|| DomainError::NotFound(format!("Movie {id}")))?
+    } else {
+        let deps = MovieResolverDeps {
+            repository: ctx.repos.movie.as_ref(),
+            metadata_client: ctx.services.metadata.as_ref(),
+        };
+        let (movie, is_new) = MovieResolver::default_pipeline()
+            .resolve(&cmd.input, &deps)
+            .await?;
+        if is_new {
+            ctx.repos.movie.upsert_movie(&movie).await?;
+            if let Some(ext_id) = movie.external_metadata_id() {
+                let _ = ctx
+                    .services
+                    .event_publisher
+                    .publish(&DomainEvent::MovieDiscovered {
+                        movie_id: movie.id().clone(),
+                        external_metadata_id: ext_id.clone(),
+                    })
+                    .await;
+            }
+        }
+        movie
+    };
+
+    let entry = WatchlistEntry::new(user_id.clone(), movie.id().clone());
+    ctx.repos.watchlist.add(&entry).await?;
+
+    let _ = ctx
+        .services
+        .event_publisher
+        .publish(&DomainEvent::WatchlistEntryAdded {
+            user_id,
+            movie_id: movie.id().clone(),
+            movie_title: movie.title().value().to_string(),
+            release_year: movie.release_year().value(),
+            external_metadata_id: movie.external_metadata_id().map(|e| e.value().to_string()),
+            added_at: entry.added_at,
+        })
+        .await;
+
+    Ok(())
+}
+
+#[cfg(test)]
+#[path = "tests/add.rs"]
+mod tests;