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/views.py

@@ -18,6 +18,7 @@ from django.http.response import JsonResponse, HttpResponse
 from django.views.decorators.csrf import csrf_exempt  # new
 from django.contrib.auth import logout, authenticate, login
 from django.contrib.auth.decorators import login_required
+from django.urls import reverse
 import stripe
 import logging
 
@@ -122,6 +123,55 @@ def useful_links(request):
     return render(request, "schasite/useful_links.html", {"links": useful_links})
 
 
+PUBLIC_SITEMAP_ROUTES = (
+    ("index2", "weekly", "1.0"),
+    ("about_us2", "monthly", "0.8"),
+    ("calendar2", "weekly", "0.8"),
+    ("dues2", "monthly", "0.9"),
+    ("membership_form2", "monthly", "0.9"),
+    ("scha_board2", "monthly", "0.7"),
+    ("useful_links2", "monthly", "0.6"),
+    ("newsletters2", "monthly", "0.5"),
+)
+
+
+def robots_txt(request):
+    sitemap_url = request.build_absolute_uri(reverse("sitemap_xml"))
+    return render(
+        request,
+        "schasite/robots.txt",
+        {"sitemap_url": sitemap_url},
+        content_type="text/plain",
+    )
+
+
+def sitemap_xml(request):
+    urls = [
+        {
+            "loc": request.build_absolute_uri(reverse(name)),
+            "changefreq": changefreq,
+            "priority": priority,
+        }
+        for name, changefreq, priority in PUBLIC_SITEMAP_ROUTES
+    ]
+    return render(
+        request,
+        "schasite/sitemap.xml",
+        {"urls": urls},
+        content_type="application/xml",
+    )
+
+
+def llms_txt(request):
+    site_root = request.build_absolute_uri("/")
+    return render(
+        request,
+        "schasite/llms.txt",
+        {"site_root": site_root},
+        content_type="text/plain; charset=utf-8",
+    )
+
+
 def index2(request):
     return render(request, "schasite/index2.html", {})