chore: harden common errors, fastembed blocking, and ingest ownership

Run FastEmbed inference on spawn_blocking, propagate Surreal take
failures,
add AppError::internal and typed ingest/embedding parse errors, and take
owned file lists in ingestion payload construction.

common/src/storage/db.rs

@@ -124,7 +124,7 @@ impl SurrealDbClient {
             .load_files(&MIGRATIONS_DIR)
             .up()
             .await
-            .map_err(|e| AppError::InternalError(e.to_string()))?;
+            .map_err(AppError::internal)?;
 
         Ok(())
     }