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/bundle-file.ts

@@ -5,8 +5,9 @@ import { LOCAL_BUNDLE_EXT, LOCAL_BUNDLE_EXTS } from "./classname";
 /**
  * Local furniture-bundle lookups.
  *
- * Bundles are written as `<classname>.hab`, but a large body of pre-existing
- * assets is still `.nitro`, so every existence check has to consider both. These
+ * New bundles are written with the configured primary extension
+ * (`FURNI_BUNDLE_EXT`, `.hab` by default), but a large body of pre-existing
+ * assets may be `.nitro`, so every existence check has to consider both. These
  * helpers are the single place that knows that, which keeps "is this item's
  * asset present?" from silently flipping to `false` for legacy files and the
  * nitro-cleanup scan from classifying them as fake leftovers.