feat: add statsd counters for signup funnel (account created + email verified, by device)

[Sadashii]

Closes #

This PR adds statsd counters to measure signup funnel dropoff — specifically, how many users who create an account actually complete email verification. We also track device type (mobile vs desktop) at both steps to test a hypothesis that the link-only email verification flow causes disproportionate mobile dropoff.

Background / Hunch: When a user signs up on mobile and hasn't logged into their email client on that device, clicking a verification link opens a new browser session where they may not have context, or worse, they may have to switch apps entirely. This friction likely causes a meaningful portion of signups to bounce before verifying. Adding device-aware counters lets us validate (or disprove) this before investing in an OTP-based alternative verification flow.

Technical

Added to openlibrary/plugins/upstream/account.py:

• _MOBILE_UA_RE — compiled regex to classify User-Agent as mobile or desktop (no new dependencies)
• get_device_label() — thin helper that reads HTTP_USER_AGENT from web.ctx.env

Counters (via openlibrary.core.stats.increment):

Counter	Fires when
ol.account.created	IA account created successfully
ol.account.created.mobile / .desktop	same, split by UA
ol.account.email_verified	email link clicked and activation succeeds
ol.account.email_verified.mobile / .desktop	same, split by UA

In Grafana: email_verified / created gives the overall funnel %. Comparing .mobile vs .desktop sub-rates will show whether mobile users are dropping off disproportionately at the verification step.

Note: previously, admin_range__members in admin/numbers.py counted new accounts daily, but only post-verification (a /type/user record isn't created until first login after activation). There was no counter for attempted signups, so there was no way to compute a funnel rate. These counters close that gap.

Testing

No new behaviour — purely additive observability. Existing tests pass unchanged.

To manually verify locally:

1. Register a new account at /account/create
2. Check statsd/Graphite for ol.account.created and ol.account.created.{mobile,desktop}
3. Click the verification link in the email
4. Check for ol.account.email_verified and ol.account.email_verified.{mobile,desktop}

Screenshot

N/A — no UI changes.

Stakeholders

@mek

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[mekarpeles]

As noted in our 1:1 conversation, statsd + devices might be challenging

It might make more sense to use matomo (which already has device type)

The question becomes much simpler; how can we trigger a matomo event on the page or redirect triggered by the activate endpoint