Mark /api/caption uncacheable

Same date can legitimately draft different captions call to call (goonk's
git/spotify state changes through the day, phrasing itself is randomized),
but nothing told any proxy/CDN layer between the browser and this
response that it shouldn't cache by URL. Explains the deployed-vs-goonk
staleness discrepancy: goonk's own endpoint may have different (or no)
caching rules than whatever fronts postcard.

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

server/src/routes.js

@@ -14,7 +14,13 @@ const ALLOWED_MIME = new Set(['image/jpeg', 'image/png', 'image/webp'])
 
 // GET /api/caption?date=YYYY-MM-DD — draft a one-line caption from that
 // day's context. Editable client-side before it's ever composited in.
+// Explicitly uncacheable: the same date can legitimately draft a
+// different caption call to call (goonk's underlying git/spotify state
+// changes through the day, and the phrasing itself is randomized), so any
+// browser/CDN/reverse-proxy layer caching this by URL would silently
+// serve stale activity data.
 apiRouter.get('/caption', async (req, res) => {
+  res.setHeader('Cache-Control', 'no-store')
   const date = /^\d{4}-\d{2}-\d{2}$/.test(req.query.date) ? req.query.date : new Date().toISOString().slice(0, 10)
   const caption = await draftCaption(date)
   res.json({ caption })