Umami decommission: gated removal of the /stats Umami path + beacon

[JSONbored]

Part of epic #8286 (Phase 4 — Sunset). Gated — mirrors JSONbored/metagraphed#7767.

Gates

• PostHog web analytics live through the first-party proxy (its sub-issue closed) with real pageview data flowing.
• The web-analytics dashboard (insights sub-issue) answers the questions the Umami dashboard answers today — verified side by side, recorded here.

Removal checklist

• apps/loopover-ui/src/lib/analytics-proxy.ts: remove the Umami upstream + /stats/api/send forwarding (or reduce the module to the PostHog-only routes, keeping the allowlist posture and its tests).
• Remove the local Umami beacon from the UI; sweep for the Umami env/config references.
• Docs/comment sweep for the Umami architecture notes (analytics-proxy.ts header, server.ts comments).

The self-hosted Umami instance itself is operator infrastructure — the maintainer shuts it down personally after repo-side removal lands; not a PR concern beyond removing what points at it.

[linear-code]

JSO-5744