fix: set PYTHONPATH=/app so alembic can import app package

When the alembic console-script entry point runs, Python sets sys.path[0]
to the entry point directory (/usr/local/bin/), not the WORKDIR. Without
PYTHONPATH=/app, `from app.models import Base` in alembic/env.py fails
with ModuleNotFoundError. uvicorn is unaffected as it adds CWD to
sys.path itself.

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

backend/Dockerfile

@@ -1,6 +1,7 @@
 FROM python:3.12-slim
 
 WORKDIR /app
+ENV PYTHONPATH=/app
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
     gcc libpq-dev \