feat: fix content sharing — nginx proxy, file path resolution, catalog filtering

- Add /content and /dwn proxy locations to nginx config (both HTTP and HTTPS)
  so peer requests reach the backend instead of the SPA catch-all
- Update content_file_path() to check FileBrowser data dir as fallback when
  files aren't in the dedicated content/files/ directory
- Populate size_bytes from actual file metadata in content.add
- Filter out availability:nobody items from the public catalog endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/archipelago/src/api/handler.rs

@@ -348,10 +348,11 @@ impl ApiHandler {
     async fn handle_content_catalog(config: &Config) -> Result<Response<hyper::Body>> {
         match content_server::load_catalog(&config.data_dir).await {
             Ok(catalog) => {
-                // Only expose public metadata, not file paths
+                // Only expose public metadata for available items
                 let items: Vec<serde_json::Value> = catalog
                     .items
                     .iter()
+                    .filter(|i| !matches!(i.availability, content_server::Availability::Nobody))
                     .map(|i| {
                         serde_json::json!({
                             "id": i.id,