Templates outside the Docker build context crash the whole app, not one route

Adding server-rendered templates in a sibling directory to the app package is invisible in local dev and fatal in the image: if the Dockerfile is COPY app ./app with the build context set to the app subdirectory, a template dir at repo root never enters the image, and a module-level Jinja Environment or FileSystemLoader pointed at it raises during import. Because the import happens at startup, the container crash-loops and every route dies, not just the new page. Two fixes together: package the template dir into the image, and make the loader resolution lazy with a graceful 503 plus a loud log so a missing template can never take down unrelated endpoints. Verify by building the actual image rather than trusting a local uvicorn run.