Fix NoneType subscript error in record_run

- Add get_asset_by_name_tx for transaction-aware asset lookup
- Use transaction connection instead of separate connection
- Prevents race condition where asset might not be visible

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server.py

@@ -2194,8 +2194,8 @@ async def record_run(req: RecordRunRequest, user: User = Depends(get_required_us
                     await db.create_asset_tx(conn, input_asset)
 
             # Check if output already exists (by content_hash) - return existing if so
-            if await db.asset_exists_by_name_tx(conn, output_hash):
-                existing = await db.get_asset(output_hash)
+            existing = await db.get_asset_by_name_tx(conn, output_hash)
+            if existing:
                 logger.info(f"record_run: Output {output_hash[:16]}... already exists")
                 return {"asset": existing, "activity": None, "existing": True}