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/tests.py

@@ -89,6 +89,49 @@ class PublicPageTests(TestCase):
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, "City of Wheaton")
 
+    def test_public_pages_include_seo_metadata(self):
+        pages = [
+            ("index2", "Home | Stonehedge"),
+            ("about_us2", "About Us | Stonehedge"),
+            ("calendar2", "Calendar | Stonehedge"),
+            ("dues2", "Pay Dues | Stonehedge"),
+            ("membership_form2", "Join Today | Stonehedge"),
+            ("scha_board2", "SCHA Board | Stonehedge"),
+            ("useful_links2", "Useful Links | Stonehedge"),
+        ]
+
+        for name, title_fragment in pages:
+            with self.subTest(page=name):
+                response = self.client.get(reverse(name))
+                self.assertEqual(response.status_code, 200)
+                self.assertContains(response, f"<title>{title_fragment}")
+                self.assertContains(response, 'name="description"')
+                self.assertContains(response, 'property="og:title"')
+                self.assertContains(response, 'rel="canonical"')
+                self.assertContains(response, 'id="main-content"')
+                self.assertContains(response, "Skip to main content")
+
+    def test_robots_txt(self):
+        response = self.client.get(reverse("robots_txt"))
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response["Content-Type"].split(";")[0], "text/plain")
+        self.assertContains(response, "Sitemap:")
+        self.assertContains(response, "Disallow: /admin/")
+
+    def test_sitemap_xml(self):
+        response = self.client.get(reverse("sitemap_xml"))
+        self.assertEqual(response.status_code, 200)
+        self.assertIn("xml", response["Content-Type"])
+        self.assertContains(response, reverse("index2"))
+        self.assertContains(response, reverse("dues2"))
+
+    def test_llms_txt(self):
+        response = self.client.get(reverse("llms_txt"))
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(response["Content-Type"].split(";")[0], "text/plain")
+        self.assertContains(response, "Stonehedge Community Homeowners Association")
+        self.assertContains(response, reverse("membership_form2").lstrip("/"))
+
 
 @override_settings(
     RECAPTCHA_PUBLIC_KEY="test-public-key",