Purge the private docs Fastly service for private repositories

[ericmj]

Makes hexdocs cache purging repository-aware. Private repositories' docs are now edge-cached by the new docs-private Fastly compute service, so publish/delete of private docs must purge that service (:fastly_hexdocs_private) rather than the public one. The public hexpm path is unchanged.

• purge/2 selects :fastly_hexdocs_private for non-hexpm repositories via public?/1
• purge_hexdocs_cache/4 is generalized from hardcoded "hexpm" to any repository
• HEXDOCS_FASTLY_HEXDOCS_PRIVATE (the prod docs-private service id) is wired into runtime config; the Fastly adapter resolves it generically
• the local CDN adapter records purges so both branches are covered by tests

Deploy ordering: the prod deployment config map must already provide HEXDOCS_FASTLY_HEXDOCS_PRIVATE (fail-fast on boot) — set via the hexpm-ops docs-private service.