Add runtime-resolution of controlled vocabularies

[mobiusklein]

Discussion on 26/2/2025 concluded we want both compile time backups and runtime updated controlled vocabularies (CV) databases.

• This functionality should be behind a feature flag. When not enabled, you only get the pre-baked databases.
• This functionality should automatically fall back to the pre-baked databases when the network connection is unavailable.
• (Opinion) This functionality should provide a sync API by default to avoid async runtime coupling insanity. An async/sync deliberate entry-point to this fetching machinery could be available for application lifecycle management.
• (Question) This functionality should support caching the latest CV in platform-appropriate locations on the file structure (e.g. using https://crates.io/crates/platform-dirs) or using an application specific mechanism by providing a path.
• Operations that depend upon the database should provide two versions, one to use a specific CV database instance, and another to use the "global" instance.

This could be done either concurrently with the greater restructuring in #56 or as a follow-up task.