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

backend/app/main.py

@@ -78,6 +78,11 @@ def _migrate_add_missing_columns(sync_conn):
         ("station_config", "color_black", sa.String(7), "#1a1816"),
         # Multi-admin support: password_hash (nullable — existing rows stay NULL)
         ("users", "password_hash", sa.String(255), None),
+        # Social media columns
+        ("station_config", "social_facebook", sa.String(500), ""),
+        ("station_config", "social_twitter", sa.String(500), ""),
+        ("station_config", "social_instagram", sa.String(500), ""),
+        ("station_config", "social_youtube", sa.String(500), ""),
     ]
 
     # Determine dialect