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>

package.json

@@ -32,6 +32,7 @@
   "license": "MIT",
   "devDependencies": {
     "@cloudflare/workers-types": "4.20260522.1",
+    "@types/escape-html": "^1.0.4",
     "@types/mailparser": "^3.4.6",
     "@types/rss": "^0.0.32",
     "@vitest/coverage-v8": "4.1.7",
@@ -50,8 +51,10 @@
   },
   "dependencies": {
     "@hono/zod-validator": "^0.8.0",
+    "escape-html": "^1.0.3",
     "feed": "5.2.1",
     "hono": "4.12.22",
+    "linkedom": "^0.18.12",
     "postal-mime": "^2.7.4",
     "zod": "4.4.3"
   }