W8: Refresh link-preview metadata after publishing a message #91

Split out of #64 (see PR #90). APIClient.refreshMessageMetadata(messageId:) is implemented and
tested but has no caller; #64 kept it rather than deleting it because there is a real use, but
wiring it changes the publish path and deserved its own change.

What the web does

POST /api/messages/{id}/metadata fetches OpenGraph metadata for a message's links. The web fires
it after publishing (components/messages/MessageInput.tsx:511) and again after an edit,
fire-and-forget with a swallowed failure, so a new post's link previews populate without a reload.

iOS today

ComposeView renders a live preview while composing, but nothing refreshes metadata on the server
after the message is created — so the link preview can be missing on the feed until something else
populates it.

Build this

• Call refreshMessageMetadata(messageId:) after a successful publish in ComposeView,
  fire-and-forget, only when the posted content contained at least one URL.
• Swallow every failure — it is a secondary datum and must never surface an error on the publish
  path or block the success alert.
• Do not call it from the edit path until Bug: editing a message does nothing — PATCH /api/messages/{id} is the reschedule route, not an edit route #76 is resolved; editing a message does not work.
• Remove the "no caller yet" comment on the function once wired.

Acceptance criteria

• A newly posted message with a link shows its preview in the feed without a manual refresh.
• A message with no links issues no request.
• A failure is invisible to the user and the post still succeeds.
• Unit test for the "no links ⇒ no request" guard.

Files: Views/ComposeView.swift, Services/APIClient.swift:370.