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/diary/commands.rs

@@ -0,0 +1,28 @@
+use chrono::NaiveDateTime;
+use uuid::Uuid;
+
+pub struct MovieInput {
+    pub movie_id: Option<Uuid>,
+    pub external_metadata_id: Option<String>,
+    pub manual_title: Option<String>,
+    pub manual_release_year: Option<u16>,
+    pub manual_director: Option<String>,
+}
+
+pub struct LogReviewCommand {
+    pub user_id: Uuid,
+    pub input: MovieInput,
+    pub rating: u8,
+    pub comment: Option<String>,
+    pub watched_at: NaiveDateTime,
+}
+
+pub struct DeleteReviewCommand {
+    pub review_id: Uuid,
+    pub requesting_user_id: Uuid,
+}
+
+#[derive(Clone)]
+pub struct SyncPosterCommand {
+    pub movie_id: Uuid,
+}