Admin-only diagnostic for the signup funnel (BRIEF-LP-000)

Report 24h signup-funnel counts for post-deploy verification.

BRIEF-LP-000 §6.5 / AC-LP0-03 — answers the question "is the signup_started → signup_completed wiring live in this process?" without requiring a PostHog API key at runtime.

Returns: {"signup_started_24h": int, "signup_completed_24h": int, "ratio": float | None, "events_fully_instrumented": bool}

Authorization: requires the caller's email to appear in ADMIN_EMAILS. 401 for unauthenticated, 403 for authenticated non-admin.

Trade-off (BRIEF §6.5 OQ #1): this endpoint reads a PER-WORKER, PER-PROCESS in-memory counter. Counts RESET on container restart and do NOT aggregate across worker pods. For wiring verification ("did the code path fire at all in this process in the last 24h?") that's the right signal. For precise PostHog-truth windowing, swap to a HogQL backend in a follow-up — see BRIEF-LP-000 §6 "Deferred to dev" and OQ #1 in §9.