Show workshop/event details in link previews

[mroderick]

Show workshop/event details in link previews

Closes #2353.

Some of this issue was already fixed by #2371 (ec84f05), which added workshop-specific Open Graph tags. This PR closes the remaining gaps:

• Fix the sponsor image on workshop pages. The partial called workshop.host.try(:image_url), but Sponsor has no image_url method, so og:image always fell back to the generic codebar social image. It now uses the host sponsor's avatar.url.
• Add Open Graph tags to event pages (events#show), which previously fell back to the generic shared tags. Event previews now show the event name, description, venue image, and timing.
• Add a social_image_url helper in ApplicationHelper shared by both partials, which produces an absolute URL for og:image and falls back to the codebar social image.

Meetings were deliberately left out of scope.

Testing

• New feature specs assert the rendered og: tags for workshops (host image and fallback image) and events (title, image, description, start time).
• Full suite: bundle exec parallel_rspec spec/ -n 3 — 1443 examples, 0 failures.
• RuboCop clean on changed Ruby files; haml-lint clean on changed HAML (remaining warnings in events/show.html.haml pre-date this PR).