PostHog error tracking for review-enrichment (keep the discovery-index mirror in lockstep)

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Part of epic #8286 (Phase 1 — Errors).

• Blocked by Provision a loopover-owned Posthog project for the hosted fleet #7875.

Current state

review-enrichment/src/sentry.ts — the review-enrichment service's error tracking, the module packages/discovery-index/src/sentry.ts explicitly mirrors ("deliberately mirrors review-enrichment/src/sentry.ts's shape"). Same DSN-gated init, scrub-then-capture, fingerprint/level/tag structure, plus its own scripts/validate-sentry-release.mjs / scripts/validate-sourcemaps.mjs release validators.

Deliverables

• Same swap as the discovery-index sub-issue, kept deliberately in lockstep with it (the two modules mirror each other today; keep the mirror after the swap so future maintenance stays one mental model): $exception via posthog-node, redaction preserved, release/sourcemap validators repointed at PostHog artifacts.
• Points at the loopover-owned project (Provision a loopover-owned Posthog project for the hosted fleet #7875) under this service's own secret injection.
• Sentry path stays in parallel until the decommission gate.
• Tests: parity with the existing sentry tests for this service.

Boundaries

• Do this after (or together with) the discovery-index swap and copy its reviewed shape — don't let the two mirrors drift.

Resources

• The discovery-index sub-issue (pattern-setter for this pair)
• https://posthog.com/docs/error-tracking/capture

[linear-code]

JSO-5733

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Landed as a straight replacement, not parallel-run (see the strategy-correction comment on the epic, #8286) — REES's Sentry code is fully removed, not left running alongside PostHog.

Deleted: `src/sentry.ts`, `scripts/validate-sentry-release.mjs`, the three `sentry-*.test.ts` files, `@sentry/node`/`@sentry/cli` from package.json.

Added: `src/posthog.ts` (mirrors the old sentry.ts's shape 1:1, including `captureAnalyzerDegradationPostHog`'s #5010 grouping-by-WHY behavior), `scripts/validate-posthog-release.mjs` (same narrower-than-Sentry release check as discovery-index's — symbol-set presence + no `failure_reason`, no commits/deploys/finalize concept to check), and three `posthog-*.test.ts` files with equivalent coverage (1357/1357 passing, full suite).

No Railway references anywhere — this repo doesn't deploy on Railway anymore. Release/environment resolution is now plain `POSTHOG_RELEASE`/`POSTHOG_ENVIRONMENT`/`POSTHOG_COMMIT_SHA` with no platform-specific fallback derivation (previously `RAILWAY_GIT_COMMIT_SHA`/`RAILWAY_DEPLOYMENT_ID`/`RAILWAY_ENVIRONMENT_NAME`). README's "Deploy (Railway)" section is now a generic "Deploy standalone" section; `docker-compose.yml`'s rees profile comment and the self-hosting-rees docs page are updated to reference `POSTHOG_API_KEY` instead of `SENTRY_DSN` for the same cross-wire-avoidance reasoning that applied to Sentry.

`@posthog/cli` is pinned to an exact version (`0.9.1`, no caret) rather than a range — its postinstall script downloads a platform binary over HTTPS with no checksum/signature verification (same category of risk `@sentry/cli` already carried in this codebase via its own installer, just now audited and pinned explicitly).