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/commands.rs

@@ -0,0 +1,17 @@
+use uuid::Uuid;
+
+pub struct UpdateProfileCommand {
+    pub user_id: Uuid,
+    pub display_name: Option<String>,
+    pub bio: Option<String>,
+    pub avatar_bytes: Option<Vec<u8>>,
+    pub avatar_content_type: Option<String>,
+    pub banner_bytes: Option<Vec<u8>>,
+    pub banner_content_type: Option<String>,
+    pub also_known_as: Option<String>,
+}
+
+pub struct UpdateProfileFieldsCommand {
+    pub user_id: Uuid,
+    pub fields: Vec<domain::models::ProfileField>,
+}