chore: add mailgun-webhook workspace to monorepo

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -6,7 +6,8 @@
   "workspaces": [
     "web-app",
     "email-worker",
-    "shared-code"
+    "shared-code",
+    "mailgun-webhook"
   ],
   "scripts": {
     "install:all": "npm install",