Messages: use GET /api/messages/{id}/metadata for lightweight link previews #26

Part of #17 — Message composer & feed parity gaps

The app calls POST /api/messages/{id}/metadata, which fetches and attaches link
metadata. The spec also has GET /api/messages/{id}/metadata — "Returns ONLY the message's
stored link metadata (lightweight)" — and a standalone GET /api/link-metadata?url=.

Using the POST for display re-fetches server-side on every read.

• Read stored metadata with the GET; keep the POST for the compose/attach path.
• Use GET /api/link-metadata to preview a URL while composing, before the message exists.
• Respect the account's showPreviews preference (Settings epic).

Tests: GET path used on render; POST path used on attach.

Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.