feat: signup nudge email sequence for members without a chapter

[mroderick]

Description

Implements #2384: automated emails to members who signed up via the website but haven't subscribed to a chapter.

A bounded two-stage sequence, recorded in member_email_deliveries (typed log from #2832):

1. Signup nudge — members created 7–14 days ago with no chapter subscription, not banned
2. Follow-up — members whose nudge row is at least a month old, still no subscription, not banned

Stage eligibility is derived from the typed log rows; delivery-confirmed writes (via the EmailDelivery concern, email_type: signup_nudge / signup_nudge_followup) mean a lost send retries within the stage-1 window and no member receives the same stage twice — enforced by the (member_id, email_type) unique index. Members who subscribe at any point exit the sequence. The plan (reviewed via ce-doc-review) is at docs/plans/2026-08-31-signup-nudge-email-sequence-plan.md.

Email copy is placeholder — real copy comes via Slack (per the issue) and lands as a follow-up commit before merge.

What happens when (plain language)

Every day, one scheduled job looks at recent sign-ups and sends at most two emails — then never bothers anyone again:

• A week after someone signs up, they get a short nudge: pick a codebar chapter.
  Sent exactly once — the send is recorded in a log whose database index makes a second copy impossible, even if the job re-runs.
• One month after that nudge, if they still haven't subscribed to any chapter, they get one gentle follow-up.
• After that, silence. Even a member who never subscribes gets no third email, ever.

People who are exempt at every stage:

• anyone who subscribes to a chapter (they leave the sequence immediately and get the normal chapter welcome email instead),
• anyone banned from codebar,
• anyone who signed up more than two weeks before this ships (the sequence only looks forward — nobody gets a backfilled backlog of old nudges).

Members who signed up but never finished the signup form are included — the nudge is for everyone who created an account and walked away.

Driving it: one daily task (rake chaser:signup_nudges) that must be added to the Heroku Scheduler after merge — see the ops note above.

Ops note ⚠️

After merge, add a Heroku Scheduler entry: rake chaser:signup_nudges, daily — same mechanism as rake chaser:three_months. Without it the feature ships dark. Post-merge verification (first scheduled run creates signup_nudge rows for the current cohort): Morgan.

Steps to verify

• make test — service spec covers the full stage matrix (window edges, subscribed/banned, both rows = terminal, follow-up anchored to the nudge's send time); mailer specs cover headers, body, and typed logging; rake spec covers the entry point.