stamp-monitor: build stamp in the header, and no-cache on index.html

index.html was served with no cache headers at all, so a stale one could
pin the old bundle through any number of redeploys. The header now names
the build serving it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YLYiaPYror1vLKApYKrxTu

hostinger/prod-stack.yaml

@@ -39,7 +39,7 @@ services:
   # Bee node that saw them knows how full a batch is. Sitting next to prod_bee
   # is what makes fullness, the bucket map and batch_full alerts possible.
   stamp-monitor:
-    image: ghcr.io/ffaerber/swarm-stamp-monitor@sha256:09b33ab4e3f618144082090cfa2568c139b669f8cac8932b381fe6db18fd9a8c
+    image: ghcr.io/ffaerber/swarm-stamp-monitor@sha256:5125b63fc430e2f5c9eb97bb5cc6fb60316569468f4e3e6819ddb6ae6809cd09
     networks: [internal, public]
     volumes:
       - stamp-monitor-data:/data