Remove Needle; stop paging on nginx 4xx

Needle scored ~chance on the golden set and never gated alerts, so drop the
sidecar, proxy handlers, UI buttons, k8s container and build step. The labelled
eval sets move to cmd/triage-eval/data.

nginx 4xx (registry 401 challenges, /oauth2/auth, scanner 405s) is now dropped
in FilterActionable instead of only being discounted, so it cannot ride along
with a real 5xx. nginx 5xx still alerts.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/ML.md

@@ -1,7 +1,7 @@
 # Error detection model
 
 > Latest status, decisions and next steps: [STATUS.md](STATUS.md). Where they
-> differ (ClickHouse is now a LoadBalancer; Needle is dormant), STATUS.md wins.
+> differ (ClickHouse is now a LoadBalancer; Needle is removed), STATUS.md wins.
 
 ## Current handoff (2026-09-18)