Consider defining an enum for substitution tokens

HTML fragments in `api.toml` contain substitution tokens like `{{NAME}}`. Currently, these are referenced as string literals, but there are only a few such tokens currently. Consider defining an enum similar to ApiKey.

Advantages
- Collects the list in one place
- Compiler helps with spelling errors

Disadvantages
- One more thing to update
- Makes the code more indirect

Start by inspecting the substitution tokens in api.toml and the existing ApiKey definition. Locate every string-literal reference, then determine whether a shared enum is appropriate and what update scope it creates; the issue is complete only once that design decision and its tradeoffs are resolved.

Written by the indexing model from the issue text.