fix(seo): comprehensive SEO improvements per expert audit

- Add public/robots.txt (allow all, disallow /admin and /login, reference sitemap)
- Add public/sitemap.xml (5 public routes with change frequencies and priorities)
- Fix heading hierarchy: h2 -> h1 on about, schedule, events, contact pages
- Remove dead footer href="#" links:
  - Volunteer/Corporate Giving/Planned Giving -> routerLink to /contact with subject query param
  - Social media links -> conditional on config().social_* fields (hidden when empty)
- Fix logo ARIA: add aria-hidden="true" to decorative logo in about page divider
- Add social_facebook, social_twitter, social_instagram, social_youtube to:
  - StationConfig DB model + startup migration
  - StationConfigResponse and StationConfigUpdate schemas
  - Frontend StationConfig TypeScript interface
- Fix events "Learn More" dead link -> routerLink to /contact

src/app/contact/contact.component.html

@@ -2,7 +2,7 @@
   <div class="container">
     <div class="contact-header">
       <img src="svg/small-flower.svg" alt="" class="contact-flower" aria-hidden="true">
-      <h2>Get in <span class="text-accent">Touch</span></h2>
+      <h1>Get in <span class="text-accent">Touch</span></h1>
       <p class="section-intro">
         Have a song request, want to volunteer, or just want to say hello?
         We'd love to hear from you.