Add PYTHONPATH=/app so Hypercorn spawn workers find app module

Hypercorn spawns worker processes via multiprocessing spawn, which
starts a fresh Python interpreter without the parent's sys.path
modifications from path_setup.py. Setting PYTHONPATH=/app ensures
the worker can import app.py and all project packages.

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

Dockerfile

@@ -13,6 +13,7 @@ FROM python:3.11-slim AS base
 
 ENV PYTHONDONTWRITEBYTECODE=1 \
     PYTHONUNBUFFERED=1 \
+    PYTHONPATH=/app \
     PIP_NO_CACHE_DIR=1 \
     APP_PORT=8000 \
     APP_MODULE=app:app