Adds GET /api/v2/notifications.atom as a Huma operation producing
application/atom+xml, so the feed shows in the v2 OpenAPI spec with an
opaque XML body schema. It mirrors /feeds/notifications.atom on the wire.
Feed readers can't carry a bearer header, so the op declares an HTTP
Basic security scheme (BasicAuth) and authenticates inside the handler:
it parses the Authorization: Basic header and validates the API token
via the shared feeds.AuthenticateFeedToken, returning a 401 with a Basic
challenge on failure, then streams feeds.BuildNotificationsAtomFeed. The
path is in unauthenticatedAPIPaths so the JWT middleware lets it through.
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