fix(feed): use permalink URL as Atom entry id, strip mso-* inline styles

- Entry <id> was a non-URL string (timestamp + base64 snippet), which
  is invalid per the Atom spec; now uses the entry permalink URL which
  is both valid and stable across feed regeneration
- Strip mso-* properties from inline style attributes in extracted body
  content to eliminate the feed validator DangerousStyleAttr warning
  caused by Microsoft Office HTML in newsletter emails

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/types/index.ts

@@ -71,6 +71,7 @@ export interface WebSubSubscription {
   callbackUrl: string;
   secret?: string;
   expiresAt: number; // Unix timestamp ms
+  format?: "rss" | "atom";
 }
 
 // Declare KVNamespace for TypeScript