Rework platform billing: per-plan clocks, grace from period start

Implements BILLING.md. The architecture was sound -- snapshot-on-Due,
dated prices, asymmetric dry-run commands are all kept -- so this
fixes the three hardcoded assumptions rather than rewriting.

The real defect: grace ran from period_END, so an annual club used
the whole unpaid year plus 45 days (~410 days) before anything
switched it off. Grace now runs from the period START, and every
clock is per-plan.

- Tier -> Plan (+ TierPrice -> PlanPrice, and every FK). Migration
  0004 is hand-written: run non-interactively, makemigrations emits
  DeleteModel+CreateModel and drops every price, subscription and
  due. Its two RemoveConstraints must come first, or SQLite's
  table-rebuild tries to render a constraint over a just-renamed
  column. Verified by round-tripping real rows through it.
- Plan gains duration_months / renewal_lead_days / grace_days /
  is_trial, with CheckConstraints and a matching clean() so the form
  reports an impossible plan instead of 500ing on IntegrityError.
- Existing dues keep their stored grace_until. Re-deriving it would
  put the date in the past for every open annual period and archive
  the entire paying customer base on the next --commit run.
- Trials take their length from the trial plan's own duration_months;
  start_trial() loses its trial_months argument.
- New BillingNotice service drives a club-facing warning: every level
  on the dashboard, and on every management page once urgent.
- send_billing_reminders emails club admins, once per escalation
  level so a daily cron is not a daily email. SMTP settings are
  env-driven and provider-agnostic; the backend defaults to console.
- Paying does not auto-restore an archived club -- the control panel
  surfaces a Reactivate prompt instead, since a club can also be
  archived by hand.

DEPLOYMENT.md

@@ -117,15 +117,22 @@ docker compose up -d --no-deps web
 
 ## Scheduled jobs
 
-Three commands need to run on a schedule. Put them on the **host**, not in a container, and on
+Four commands need to run on a schedule. Put them on the **host**, not in a container, and on
 **exactly one node** when you have several — three nodes archiving the same club is three
 emails to the same club.
 
 ```cron
+# Bill: remind club admins about outstanding platform fees. Dry-run by default, same as the
+# archive job below — this one mails paying customers, so --commit is opt-in. Reminders go
+# once per escalation level, not once per run, so a daily cron is not a daily email.
+0 5 * * *  cd /srv/rosterchief && docker compose run --rm web python manage.py send_billing_reminders --commit
+
 # Bill: archive clubs unpaid past their grace period.
 # Run it WITHOUT --commit for the first week and read the output. The flag exists because
 # this switches off paying customers: a bad clock or a bad cron should cost you an email,
-# not a morning of angry clubs.
+# not a morning of angry clubs. Since grace now runs from the period START rather than its
+# end (see BILLING.md §3), this job is load-bearing in a way it never used to be — a club
+# is archivable ~60 days after being invoiced, not ~410. Re-do the dry-run week.
 0 6 * * *  cd /srv/rosterchief && docker compose run --rm web python manage.py archive_overdue_clubs --commit
 
 # Events: extend recurring series so the calendar never runs dry.