⚡ Bolt: Offload blocking file I/O to threadpool in field officer uploads

.jules/bolt.md

@@ -93,3 +93,7 @@
 ## 2026-05-20 - Joined Queries for Integrity Verification
 **Learning:** Performing multiple sequential database queries to verify cryptographically chained records (e.g., fetching a record and then its associated token/metadata from another table) introduces unnecessary latency and increases database load.
 **Action:** Consolidate associated data retrieval into a single SQL `JOIN` query within the verification hot-path. This reduces database round-trips and improves end-to-end latency for blockchain-style integrity checks.
+
+## 2026-05-26 - Async File I/O in Field Officer Uploads
+**Learning:** Saving visit images synchronously in the fastapi async endpoint blocks the main event loop, significantly increasing tail latency.
+**Action:** Wrap blocking synchronous File I/O operations like `f.write()` in `run_in_threadpool` to offload them to a separate thread, keeping the event loop responsive for other requests.

backend/routers/field_officer.py

@@ -5,6 +5,7 @@
 """
 
 from fastapi import APIRouter, Depends, HTTPException, UploadFile, File, Form, Response
+from fastapi.concurrency import run_in_threadpool
 from sqlalchemy.orm import Session
 from sqlalchemy import func, case
 from typing import List, Optional
@@ -337,8 +338,11 @@ async def upload_visit_images(
             file_path = os.path.join(VISIT_IMAGES_DIR, safe_filename)
 
             # Save file
-            with open(file_path, 'wb') as f:
-                f.write(content)
+            def _save_file():
+                with open(file_path, 'wb') as f:
+                    f.write(content)
+
+            await run_in_threadpool(_save_file)
 
             # Store relative path
             relative_path = os.path.join("data", "visit_images", safe_filename)