Add URL prefix mounting for proxy deployments

[Ulthran]

Motivation

• Enable running the app locally for development while also allowing deployment behind a reverse proxy that exposes the app at a URL prefix.
• Provide a simple configuration via an environment variable so deployment manifests (e.g. podman/docker/k8s) can control the mounted prefix without changing code.

Description

• Add a _normalize_url_prefix helper that normalizes SAMPLE_REGISTRY_URL_PREFIX and returns a cleaned prefix string.
• When SAMPLE_REGISTRY_URL_PREFIX is set, mount the Flask app at that prefix using werkzeug.middleware.dispatcher.DispatcherMiddleware and expose the WSGI entrypoint as sample_registry.app:application, otherwise keep application = app for direct development runs.
• Preserve existing ProxyFix usage and read-only SQLite configuration; update imports and documentation by adding DispatcherMiddleware and documenting SAMPLE_REGISTRY_URL_PREFIX in README.md.

Testing

• No automated tests were executed for this change.

---

Codex Task

[Copilot]

Pull request overview

This PR adds support for mounting the Flask application under a configurable URL prefix so it can run behind a reverse proxy while preserving the existing local development workflow and read-only SQLite behavior.

Changes:

• Introduced _normalize_url_prefix and URL_PREFIX handling in sample_registry/app.py, wiring DispatcherMiddleware to expose a WSGI entrypoint sample_registry.app:application that mounts the app at SAMPLE_REGISTRY_URL_PREFIX when set.
• Kept ProxyFix and read-only SQLite configuration intact while ensuring application falls back to the bare app in non-prefixed (local dev) setups.
• Updated README.md to document SAMPLE_REGISTRY_URL_PREFIX and the sample_registry.app:application WSGI entrypoint for proxy deployments.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sample_registry/app.py	Adds URL prefix normalization, configures DispatcherMiddleware to mount the Flask app under an optional SAMPLE_REGISTRY_URL_PREFIX, and exposes application as the WSGI entrypoint while preserving ProxyFix and the existing dev app.run path.
README.md	Documents how to configure SAMPLE_REGISTRY_URL_PREFIX and use sample_registry.app:application when deploying behind a reverse proxy at a URL prefix.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

_normalize_url_prefix is annotated to accept str, but it is immediately used with os.getenv("SAMPLE_REGISTRY_URL_PREFIX"), which can return None; this mismatched type annotation can cause static type checkers (e.g., mypy) to flag an error and makes the intended contract less clear. Consider updating the parameter type to Optional[str] (and importing it if needed) so the signature accurately reflects the possible inputs.