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.

README.md

@@ -594,7 +594,7 @@ URLs look like `https://<hotel>/imaging/avatarimage?figure=hd-180-1.ch-210-66&im
 ### Requirements (host)
 
 - Docker (daemon with `build.network: host`, same as the CMS build — this host disables Docker iptables).
-- An nginx that serves `/gamedata/` (FigureData/FigureMap/EffectMap…) and `/gamedata/bundled` (furniture bundle assets — `.hab`, the extension imports write and this deployment's client requests, plus any legacy `.nitro` still on disk) over HTTP so the renderer can fetch them. The compose file points at `host.docker.internal:8081`.
+- An nginx that serves `/gamedata/` (FigureData/FigureMap/EffectMap…) and `/gamedata/bundled` (furniture bundle assets — `.hab` by default, the extension imports write and this deployment's client requests, plus any legacy `.nitro` still on disk; choose the written extension with `FURNI_BUNDLE_EXT`) over HTTP so the renderer can fetch them. The compose file points at `host.docker.internal:8081`.
 
 ### Configuration — `/docker/Polaris-imager/.env`