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/scha_board2.html

@@ -2,7 +2,7 @@
 {% load static  %}
 
 {% block pagetitle %}
-<title>Stonehedge Community Homeowners Association</title>
+<title>SCHA Board | Stonehedge Community Homeowners Association</title>
 <style type="text/css">
 /* Board Page Specific Styles */
 .card-header h4 {
@@ -35,15 +35,23 @@
 </style>
 {% endblock %}
 
+{% block meta_tags %}
+<meta name="description" content="Meet the volunteer Stonehedge Community Homeowners Association board officers serving Wheaton residents.">
+{% endblock %}
+{% block og_title %}SCHA Board | Stonehedge Community Homeowners Association{% endblock %}
+{% block og_description %}Meet the volunteer Stonehedge Community Homeowners Association board officers serving Wheaton residents.{% endblock %}
+{% block twitter_title %}SCHA Board | Stonehedge Community Homeowners Association{% endblock %}
+{% block twitter_description %}Meet the volunteer Stonehedge Community Homeowners Association board officers serving Wheaton residents.{% endblock %}
+
 {% block content %}
-<main class="py-5">
+<div class="py-5">
     <div class="container">
         <!-- Board Information -->
         <div class="row justify-content-center mb-5">
             <div class="col-lg-10">
                 <div class="card shadow-sm">
                     <div class="card-body text-center">
-                        <h3 class="text-success mb-3">2025-2026 Stonehedge Community Homeowners Association Board</h3>
+                        <h1 class="h3 text-success mb-3">2025-2026 Stonehedge Community Homeowners Association Board</h1>
                         <p>The Stonehedge Community Homeowners Association is governed by a volunteer board of directors elected by the community members. Board members serve two-year terms and are responsible for overseeing the community's operations, finances, and enforcement of covenants.</p>
                     </div>
                 </div>
@@ -98,5 +106,5 @@
 
         
     </div>
-</main>
+</div>
 {% endblock %}