Add edge cache headers to all API routes to reduce Vercel function invocations

All API routes serve static content that only changes at deploy time, but had
0% cache hit rate. Adding s-maxage=86400 lets Vercel's CDN cache responses at
the edge, which should take the ~28K daily API requests from 0% to ~99% cache.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/command-source/[id].js

@@ -23,6 +23,7 @@ export default function handler(req, res) {
     }
 
     const content = readFileSync(commandPath, "utf-8");
+    res.setHeader("Cache-Control", "public, s-maxage=86400, stale-while-revalidate=3600");
     res.status(200).json({ content });
   } catch (error) {
     console.error("Error in /api/command-source:", error);