SEO enhancement and agentic browsing updates (#18) (#27)

## Summary
Closes #18.

- Add per-page titles, meta descriptions, Open Graph/Twitter tags, and canonical URLs on public `base2.html` pages
- Improve semantic structure: skip link, primary nav ARIA, single `<main id="main-content">`, heading hierarchy, descriptive image alts
- Publish `/robots.txt`, `/sitemap.xml`, and `/llms.txt` (linked from site head via `rel="alternate"`)
- Agentic/a11y fixes: membership checkbox label associations, ZIP `name` attribute, layout stability (image aspect-ratio / min-height), footer year without `document.write`
- WebMCP deferred — membership + Stripe checkout remain standard HTML/Stripe flows; no audit flag justified custom MCP surface yet

## Lighthouse / agentic notes
Baseline Lighthouse runs still need Chrome Canary export on staging/prod. Structural SEO/a11y items from the ticket are implemented so follow-up audits can capture before/after scores.

Expected audit wins vs prior minimal `charset`+`viewport` head:
- SEO: unique titles/descriptions, crawl directives, sitemap
- Agentic/a11y: landmarks, labels, CLS-oriented image sizing

## Test plan
- [x] `uv run python manage.py test schasite` (34 passed)
- [ ] Spot-check Home/About/Calendar/Dues/Membership/Board/Links in browser
- [ ] Confirm `/robots.txt`, `/sitemap.xml`, `/llms.txt` on deployed host
- [ ] Run Lighthouse SEO + Agentic browsing on key pages and note before/after pass ratio on the issue

Reviewed-on: #27

schasite/templates/schasite/robots.txt

@@ -0,0 +1,18 @@
+User-agent: *
+Allow: /
+
+Disallow: /admin/
+Disallow: /login/
+Disallow: /signup/
+Disallow: /dashboard/
+Disallow: /directory/
+Disallow: /posts/
+Disallow: /posts_create/
+Disallow: /profile/
+Disallow: /password_reset/
+Disallow: /set_password/
+Disallow: /config/
+Disallow: /create-checkout-session/
+Disallow: /webhook/
+
+Sitemap: {{ sitemap_url }}