feat(assets): choose furniture bundle ext via FURNI_BUNDLE_EXT

Bundle the remaining imaging hardening and the host/CI fixes from the
"Doe alles" pass into one release.

Furniture:
- Resolve the local bundle extension from FURNI_BUNDLE_EXT (".hab" default,
  ".nitro" for a stock Nitro client). The configured extension is written by
  imports/repairs and preferred on lookup; the other stays a legacy fallback.

Imaging:
- Avatar proxy sends s-maxage=86400 + stale-while-revalidate (300/3600 for the
  fallback path) so the edge can hold a render a day.
- Cloudflare cache rule now covers /api/imaging/ (verified MISS->HIT live).
- Document that bare /imaging is shadowed by the edge proxy to the renderer.

Ops/CI:
- docker-update.sh skips a CI-managed host (exit 0) instead of failing cron.
- ci-deploy.sh runs the cross-browser imaging gate (chromium+firefox+webkit)
  against the candidate before cutover; DEPLOY_SKIP_CROSS_BROWSER=1 as escape.
- scripts/imager-health-alert.sh alerts (Discord) on imager up/down transitions;
  runs from cron every 5 minutes.

Tests:
- Furniture bundle-ext resolver + FURNI_BUNDLE_EXT wiring.
- Imaging performance budget (bytes + warm latency).
- classifyDdos: /api/imaging/* stays in the api bucket.

src/app/api/imaging/avatar/route.ts

@@ -21,12 +21,16 @@ function cacheControlFor(source: string): string {
 	switch (source) {
 		case "fallback":
 		case "cache-stale":
-			// Reuse the render briefly but revalidate the upstream soon.
-			return "public, max-age=300";
+			// Reuse the render briefly but revalidate the upstream soon. The
+			// shared `s-maxage` lets the edge (Cloudflare) hold it for an hour
+			// without pinning a degraded fallback in the browser.
+			return "public, max-age=300, s-maxage=3600, stale-while-revalidate=3600";
 		case "unavailable":
 			return "no-cache, max-age=0";
 		default:
-			return "public, max-age=3600";
+			// s-maxage=86400 lets the edge cache a day off the origin, matching
+			// the sibling /imaging route; the URL already keys per figure.
+			return "public, max-age=3600, s-maxage=86400, stale-while-revalidate=86400";
 	}
 }