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/rpc/content.rs

@@ -31,7 +31,7 @@ impl RpcHandler {
             .and_then(|v| v.as_str())
             .unwrap_or("");
 
-        let item = ContentItem {
+        let mut item = ContentItem {
             id: uuid::Uuid::new_v4().to_string(),
             filename: filename.to_string(),
             mime_type: mime_type.to_string(),
@@ -42,6 +42,12 @@ impl RpcHandler {
             added_at: chrono::Utc::now().to_rfc3339(),
         };
 
+        // Resolve actual file size from disk
+        let file_path = content_server::content_file_path(&self.config.data_dir, &item);
+        if let Ok(metadata) = std::fs::metadata(&file_path) {
+            item.size_bytes = metadata.len();
+        }
+
         content_server::add_item(&self.config.data_dir, item.clone()).await?;
         Ok(serde_json::json!({ "item": item }))
     }