feat: add MIME plumbing for markdown (.md) format

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -19,6 +19,7 @@
     "express-rate-limit": "^8.6.1",
     "file-type": "^22.0.1",
     "mammoth": "^1.12.0",
+    "markdown-it": "^15.0.0",
     "multer": "^2.2.0",
     "pdf-lib": "^1.17.1",
     "pdfjs-dist": "^6.2.108",
@@ -27,6 +28,7 @@
     "react": "^19.2.8",
     "react-dom": "^19.2.8",
     "sharp": "^0.35.3",
+    "turndown": "^7.2.4",
     "uuid": "^14.0.1"
   },
   "devDependencies": {