Fix OpenGraph tags on all important pages

[shayesdevel]

Summary

Fixes #130 -- OpenGraph and Twitter Card meta tags were broken or missing across the site.

Problems fixed

1. Relative image URLs: og:image and twitter:image used relative paths (e.g. /assets/images/...) which social media crawlers cannot resolve. Now prefixed with SITE_FULL.
2. Wrong og:site_name: Used request.host (raw hostname) instead of SITE_TITLE (human-readable name).
3. Duplicate og:site_name: default.html and settings.html had two og:site_name tags.
4. Wrong og:type: Homepage, settings, and other non-article pages used og:type=article. Now uses website for general pages, article for posts, profile for user pages.
5. Invalid og:author: Used on non-article pages. Removed where inappropriate, kept twitter:creator where it makes sense (post/comment authors, user profiles).
6. Wrong twitter:url: Submission comment view used p.permalink instead of comment_info.permalink.
7. Missing OG tags: Error pages, login/signup forms, and the submit page had no OG tags at all.

Templates changed

• default.html -- Fixed base OG tags (type, site_name, image URLs, removed duplicates)
• submission.html -- Fixed post and comment OG tags, removed og:author
• userpage.html -- Added og:type=profile, fixed title/site_name/description
• settings.html -- Fixed all OG/Twitter tags, removed duplicates
• settings2.html -- Fixed OG/Twitter tags
• errors/error.html -- Added OG and Twitter tags
• login/authforms.html -- Added OG and Twitter tags
• submit.html -- Added OG and Twitter tags

Tests added

• test_opengraph.py: Verifies og:title, og:image, og:type, og:site_name, and Twitter tags on front page, post pages, login page, and user profiles.

Test plan

• CI tests pass
• Paste a link to the site in Discord/Slack/Twitter and verify the preview card renders correctly
• Check post links show post title and thumbnail in social previews
• Check user profile links show username and banner in social previews

Generated with Claude Code

[zorbathut]

(pasting in claude review about the things I actually care about)

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Bug: og:image on userpage is still potentially relative. u.banner_url (user.py:467-470) falls back to assetcache_path(...) when the user has no custom
banner, which returns a relative path like /assets/images/.../site_preview.webp. The PR fixes this exact problem in every other template by prepending
SITE_FULL, but missed it here. Should be content="{{SITE_FULL}}{{u.banner_url}}" — but that would double-prefix when bannerurl is already absolute. The real
fix is either making banner_url always return an absolute URL (like thumb_url does) or handling both cases in the template.

Same issue for twitter:image on userpage.

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The tests are very shallow — they check for tag presence in HTML but don't validate values:

• No test verifies that og:image URLs are actually absolute (the core bug being fixed)
• No test verifies og:site_name content is SITE_TITLE rather than request.host

These tests will catch regressions where tags are accidentally deleted, but won't catch the bugs this PR is actually fixing (relative URLs, wrong values).
Consider parsing the meta tags and asserting on content values, at least for og:image (starts with http) and og:site_name.

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(haven't actually verified this in detail, sorry if this turns out to be wrong)