fix: drop uvicorn to 1 worker to prevent SQLite CREATE TABLE race

With --workers 2, both processes run Base.metadata.create_all
simultaneously. SQLite has no IF NOT EXISTS guard in DeerFlow's
engine.py, so the second worker always fails with
"table feedback already exists" and gets killed/restarted.
Single worker eliminates the race; SQLite is single-writer anyway.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

start.sh

@@ -362,7 +362,7 @@ echo "Starting DeerFlow backend on port $BACKEND_PORT..."
     uvicorn app.gateway.app:app \
       --host 127.0.0.1 \
       --port "$BACKEND_PORT" \
-      --workers 2 \
+      --workers 1 \
     2>&1 | tee -a "$DATA_DIR/logs/backend.log"
 ) &
 BACKEND_PID=$!