Implements the sender side of https://www.w3.org/TR/webmention/. During
feed scans, external URLs found in new posts (and links added by edits)
are queued as OutgoingWebmention rows; a periodic task discovers each
target's endpoint and delivers the notification. The unique
(source, target) pair guarantees a webmention is sent at most once, no
matter how many times a feed is rescanned. Targets without an endpoint
are marked permanently, failures retry with exponential backoff, and
endpoints resolving to loopback/private addresses are rejected.
Receiving webmentions is out of scope: plain text feeds cannot
advertise an endpoint.
