Provide timestamp of first source project discovery

It might be interesting for many users of ecosyste.ms to know when an project has been discovery the first time. One example use case of this to determine when a project has been made public. The GitHub API does not provide such information and this value can differ significantly from the timestamp of the first commit, as many repositories are set to public with their first release.

For our use-case at [ClimateTriage.com](https://climatetriage.com/) the project age is what we are looking for, but I still think I could be relevant for future use cases.

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Start by reviewing the ecosyste.ms project-discovery flow and the metadata exposed for source projects, then determine where a first-discovery timestamp could be recorded and returned. Confirm how the value should differ from the first commit or release timestamp and document the completed API behavior; no implementation files or tests are named in the issue.

Written by the indexing model from the issue text.