Add CSS import/link/list/delete/content API (ticket 010)

Wires up Settings.css_entries behind a small settings API: import raw CSS
content to a file stored alongside settings.json, link external CSS
paths/URLs by reference (never deletable/copied), list all entries, delete
imported entries, and serve back imported content. CSS storage logic lives
on Settings (import_css/link_css/remove_css/css_content) with api.rs
handlers staying thin, matching the existing engine-query pattern.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/main.rs

@@ -50,7 +50,7 @@ async fn main() {
     tokio::spawn(engine::run(engine_rx));
     tokio::spawn(ingest::run(engine_tx.clone()));
 
-    let app = api::router(engine_tx, &args.static_dir, settings);
+    let app = api::router(engine_tx, &args.static_dir, settings, settings_path);
     let listener = TcpListener::bind(("0.0.0.0", args.port))
         .await
         .expect("failed to bind listener");