Create a signal for the authenticated account. Requires an API key and a bearer token with write:posts consent. author_address defaults to the wallet the token signed in with; the author must be a public registered wallet on the account that holds the subject. Subject eligibility, content rules, and a per-account, per-subject cooldown apply. Creation is not idempotent: retrying after an unknown outcome can create a second signal once the cooldown allows. The public GET may return 404 when privacy settings or moderation hide the signal.