Run alembic migrations in Docker CMD

Railway's releaseCommand wasn't executing, so run migrations
directly in the container startup command instead.

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

backend/Dockerfile

@@ -18,5 +18,5 @@ COPY . .
 # Expose port (Railway uses PORT env variable)
 EXPOSE 8000
 
-# Run the application - use shell form to expand $PORT
-CMD uvicorn app.main:app --host 0.0.0.0 --port ${PORT:-8000}
+# Run migrations then start the application
+CMD alembic upgrade head && uvicorn app.main:app --host 0.0.0.0 --port ${PORT:-8000}