perf: avoid small own clones and intermediate Vec allocations

- Derive Copy on 6 small enums (MessageRole, TaskState, StorageKind, EmbeddingBackend, PdfIngestMode, KnowledgeEntityType)
- Change create_ingestion_payload files param from Vec<FileInfo> to &[FileInfo]
- Remove 5 intermediate Vec allocations (4 embedding serialization + 1 format_history) using write! loop
- Remove 7 unnecessary .clone() calls exposed by Copy derive

html-router/src/routes/ingestion/handlers.rs

@@ -142,7 +142,7 @@ pub async fn process_ingest_form(
         input.content,
         input.context,
         input.category,
-        file_infos,
+        &file_infos,
         user.id.as_str(),
     )?;