feat(api): add docs assistant endpoint with OpenRouter integration

apps/api/wrangler.jsonc

@@ -38,4 +38,6 @@
   }
 
   // Secrets (wrangler secret put): GH_PAT, WEBHOOK_SECRET, LINK_KEY.
+  // OPENROUTER_API_KEY (the docs assistant, POST /ask) is the repo's Actions
+  // secret of that name, put by deploy.yml before every deploy.
 }