chore: replace docen with turndown and unify HTTPError handling

- Replace docen with turndown for
  HTML-to-markdown conversion in reader route
- Convert throw new Error() to HTTPError({ status: 502 }) in font CSS
  generation and Google Fonts provider for consistent error responses

Author: DemoMacro

package.json

@@ -39,14 +39,15 @@
   "devDependencies": {
     "@mozilla/readability": "0.6.0",
     "@types/node": "25.6.0",
+    "@types/turndown": "5.0.6",
     "@vercel/og": "0.11.1",
     "cheerio": "1.2.0",
-    "docen": "0.0.15",
     "ioredis": "5.10.1",
     "ipx": "4.0.0-alpha.1",
     "linkedom": "0.18.12",
     "nitro": "3.0.260311-beta",
     "playwright": "1.59.1",
+    "turndown": "7.2.4",
     "vite-plus": "0.1.16"
   }
 }