✨ Persist first-touch attribution at signup + emit signup analytics on OAuth path

[PierreBrisorgueil]

What — Store where a user came from at signup, and emit signup analytics for OAuth signups too.
Why — OAuth signups currently emit no backend signup event, and nothing persists acquisition origin server-side.

Scope

• Accept an optional attribution object in the signup payload (referrer, utm_*, landing path) — validate (Zod) + persist on the user document.
• Attach the attribution props to the backend user_signed_up analytics event.
• Fire user_signed_up + $identify on the OAuth signup path (parity with local signup).
• Config-gated; inert when analytics is disabled.

Created via /dev:issue