URL link previews now post the page's og:image as an inline m.image
thumbnail above the title/description reply. All outbound fetches in the
preview path (page scrape, image HEAD probe, image download) route through
a new SSRF/DoS-hardened safehttp client so a posted link can't steer
fetches at loopback/RFC1918/cloud-metadata IPs or OOM the parser.
Also flips the daily WOTD auto-post to opt-in (ENABLE_WOTD_POST=true)
instead of opt-out.
