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);

api/commands.js

@@ -39,6 +39,7 @@ export default function handler(req, res) {
       }
     }
 
+    res.setHeader("Cache-Control", "public, s-maxage=86400, stale-while-revalidate=3600");
     res.status(200).json(commands);
   } catch (error) {
     console.error("Error in /api/commands:", error);

api/download/[type]/[provider]/[id].js

@@ -62,6 +62,7 @@ export default function handler(req, res) {
 
     const content = readFileSync(filePath);
     const fileName = basename(filePath).replace(/[^a-zA-Z0-9._-]/g, '');
+    res.setHeader("Cache-Control", "public, s-maxage=86400, stale-while-revalidate=3600");
     res.setHeader("Content-Type", "application/octet-stream");
     res.setHeader("Content-Disposition", `attachment; filename="${fileName}"`);
     res.send(content);

api/download/bundle/[provider].js

@@ -26,6 +26,7 @@ export default function handler(req, res) {
     }
 
     const content = readFileSync(zipPath);
+    res.setHeader("Cache-Control", "public, s-maxage=86400, stale-while-revalidate=3600");
     res.setHeader("Content-Type", "application/zip");
     const safeProvider = provider.replace(/[^a-zA-Z0-9._-]/g, '');
     res.setHeader("Content-Disposition", `attachment; filename="impeccable-style-${safeProvider}.zip"`);

api/patterns.js

@@ -10,6 +10,7 @@ export default function handler(req, res) {
   try {
     // Extract patterns from SKILL.md using the shared utility
     const { patterns, antipatterns } = readPatterns(PROJECT_ROOT);
+    res.setHeader("Cache-Control", "public, s-maxage=86400, stale-while-revalidate=3600");
     res.status(200).json({ patterns, antipatterns });
   } catch (error) {
     console.error("Error in /api/patterns:", error);

api/skills.js

@@ -36,6 +36,7 @@ export default function handler(req, res) {
       }
     }
 
+    res.setHeader("Cache-Control", "public, s-maxage=86400, stale-while-revalidate=3600");
     res.status(200).json(skills);
   } catch (error) {
     console.error("Error in /api/skills:", error);