Classifiers in pyproject.toml are stale (missing 3.13)

[JohnRDOrazio]

`pyproject.toml` classifiers still list:

```toml
classifiers = [
...
"Programming Language :: Python :: 3.11",
"Programming Language :: Python :: 3.12",
...
]
```

but the project has been on Python 3.13 since #095b351 (`.python-version` → `3.13`, `Dockerfile` / `Dockerfile.prod` → `python:3.13-slim`, CI resolves 3.13 via `setup-uv`).

`requires-python = ">=3.11"` is still accurate, so installs are unaffected — this is purely PyPI metadata drift. Noticed during review of #101 where a reviewer suggested downgrading the Docker base image to match classifiers (rejected — the classifiers are wrong, not the base image).

Fix

Add `"Programming Language :: Python :: 3.13"` to the classifiers list, and decide whether 3.11 is still officially supported or should be dropped.