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/lib/furni/classname.ts

@@ -17,23 +17,61 @@ export function getSwfFileName(classname: string): string {
 	return `${getBaseClassname(classname)}.swf`;
 }
 
+/** The two extensions a furniture bundle may carry. */
+export const BUNDLE_EXT_CHOICES = [".hab", ".nitro"] as const;
+export type FurnitureBundleExt = (typeof BUNDLE_EXT_CHOICES)[number];
+
+/**
+ * Read an env value via a computed key so Next does not inline it at build time
+ * — changing it should take effect on restart, not require a rebuild (same
+ * pattern as `runtime-asset-config.ts`).
+ */
+function readBundleExtEnv(): string | undefined {
+	const name = "FURNI_BUNDLE_EXT";
+	return process.env[name]?.trim() || undefined;
+}
+
+/**
+ * Resolve the local furniture-bundle extension this deployment's client asks
+ * for. Set `FURNI_BUNDLE_EXT` in `.env` to `hab`/`.hab` or `nitro`/`.nitro`;
+ * anything missing or unrecognised falls back to `.hab`.
+ */
+export function resolveLocalBundleExt(
+	raw: string | undefined = readBundleExtEnv(),
+): FurnitureBundleExt {
+	const value = raw?.trim().toLowerCase();
+	if (!value) return ".hab";
+	const normalized = value.startsWith(".") ? value : `.${value}`;
+	return (BUNDLE_EXT_CHOICES as readonly string[]).includes(normalized)
+		? (normalized as FurnitureBundleExt)
+		: ".hab";
+}
+
 /**
  * Extension locally stored furniture bundles are written with. This deployment's
- * Nitro client requests `<classname>.hab`, so that is what new imports produce.
+ * Nitro client requests `<classname>.hab` by default, but a stock Nitro client
+ * asks for `.nitro` — choose with `FURNI_BUNDLE_EXT`. New imports and repairs
+ * produce this extension.
  */
-export const LOCAL_BUNDLE_EXT = ".hab";
+export const LOCAL_BUNDLE_EXT: FurnitureBundleExt = resolveLocalBundleExt();
 
 /**
  * Extension remote sources serve bundles with. Upstream (Habbo's CDN and every
  * configured clone source) keeps shipping `.nitro`, so download URLs are built
  * with this and only re-written to `LOCAL_BUNDLE_EXT` once they land on disk.
  */
-export const SOURCE_BUNDLE_EXT = ".nitro";
+export const SOURCE_BUNDLE_EXT = ".nitro" as const;
 
-/** Both extensions a local bundle may carry, newest convention first. */
-export const LOCAL_BUNDLE_EXTS = [LOCAL_BUNDLE_EXT, SOURCE_BUNDLE_EXT] as const;
+/**
+ * Every extension a local bundle may be stored under, the configured one first
+ * so it always wins over a same-stem legacy file.
+ */
+export const LOCAL_BUNDLE_EXTS: readonly FurnitureBundleExt[] = [
+	LOCAL_BUNDLE_EXT,
+	...BUNDLE_EXT_CHOICES.filter((ext) => ext !== LOCAL_BUNDLE_EXT),
+];
 
-/** Primary local bundle name — `.hab` wins over a legacy `.nitro`. */
+/** Primary local bundle name — the configured extension wins over the legacy one. */
 export function getNitroFileName(classname: string): string {
 	return `${getBaseClassname(classname)}${LOCAL_BUNDLE_EXT}`;
 }