feat: WebSub Atom support, HTML processing via linkedom, W3C badges

WebSub / PubSubHubbub:
- Hub now accepts both /rss/:id and /atom/:id topic URLs
- WebSubSubscription stores format ("rss" | "atom")
- notifySubscribers sends RSS or Atom XML with correct Content-Type
- verifyAndStoreSubscription sends correct topic URL per format
- CI paths-ignore docs/** to skip deploy on docs-only changes

HTML processing (linkedom + escape-html):
- New html-processor.ts: body extraction, script/iframe/object removal,
  event handler + javascript: URL stripping, mso-* style cleanup,
  plain text → <pre> with HTML escaping via escape-html
- feed-generator.ts and entries.ts use processEmailContent

Admin UI:
- W3C validation badges (Atom + RSS) on feed detail page

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/styles/components.css

@@ -954,6 +954,17 @@ table.table code {
   border-color: rgba(255, 69, 58, 0.35);
 }
 
+/* Validation badges */
+.feed-validate {
+  display: flex;
+  gap: 0.5rem;
+  margin-top: 1rem;
+}
+
+.feed-validate img {
+  display: block;
+}
+
 /* Feed and Email Lists */
 .feed-list,
 .email-list {