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/routes/entries.ts

@@ -1,6 +1,7 @@
 import { Context } from "hono";
 import { html, raw } from "hono/html";
 import { Env, FeedMetadata, EmailData } from "../types";
+import { processEmailContent } from "../utils/html-processor";
 
 export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   const feedId = c.req.param("feedId");
@@ -82,7 +83,9 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
             <dt>Date:</dt>
             <dd>${new Date(emailData.receivedAt).toUTCString()}</dd>
           </dl>
-          <div class="content">${raw(emailData.content)}</div>
+          <div class="content">
+            ${raw(processEmailContent(emailData.content))}
+          </div>
         </body>
       </html>`,
   );