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/search/execute.rs

@@ -0,0 +1,9 @@
+use crate::context::AppContext;
+use domain::{
+    errors::DomainError,
+    models::{SearchQuery, SearchResults},
+};
+
+pub async fn execute(ctx: &AppContext, query: SearchQuery) -> Result<SearchResults, DomainError> {
+    ctx.repos.search_port.search(&query).await
+}