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.

controlpanel/forms.py

@@ -4,7 +4,7 @@ from django import forms
 from django.utils.translation import gettext_lazy as _
 from waffle import get_waffle_flag_model
 
-from billing.models import DuePayment, Subscription, Tier, TierPrice
+from billing.models import DuePayment, Plan, PlanPrice, Subscription
 from club.models import Club
 from events.models import Location
 
@@ -82,51 +82,61 @@ class FlagForm(forms.ModelForm):
         }
 
 
-class TierForm(forms.ModelForm):
+class PlanForm(forms.ModelForm):
     class Meta:
-        model = Tier
-        fields = ["name", "description", "is_active"]
+        model = Plan
+        fields = ["name", "description", "duration_months", "renewal_lead_days", "grace_days", "is_trial", "is_active"]
 
 
-class TierPriceForm(forms.ModelForm):
+class PlanPriceForm(forms.ModelForm):
     class Meta:
-        model = TierPrice
+        model = PlanPrice
         fields = ["active_from", "amount"]
         widgets = {"active_from": forms.DateInput(attrs={"type": "date"})}
-        help_texts = {"active_from": _("Periods opening on or after this date are billed at this amount. Existing periods keep the amount they were billed at.")}
+        help_texts = {
+            "active_from": _(
+                "Periods opening on or after this date are billed at this amount. Existing periods keep the amount "
+                "they were billed at — including any already issued during a plan's renewal lead window, so enter a "
+                "price change before that window opens."
+            )
+        }
 
 
 class SubscriptionForm(forms.ModelForm):
-    """Put a club on a tier. The first period opens when the subscription is created."""
+    """Put a club on a plan. The first period opens when the subscription is created."""
 
     start = forms.DateField(required=False, widget=forms.DateInput(attrs={"type": "date"}), label=_("First period starts"), help_text=_("Left blank, the period starts today."))
 
     class Meta:
         model = Subscription
-        fields = ["tier", "auto_renew", "auto_archive", "notes"]
+        fields = ["plan", "auto_renew", "auto_archive", "notes"]
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        # An inactive tier still bills its existing subscriptions, but must not be picked up
-        # by a new one — which is the whole point of retiring a tier.
-        self.fields["tier"].queryset = Tier.objects.filter(is_active=True)
+        # An inactive plan still bills its existing subscriptions, but must not be picked up
+        # by a new one — which is the whole point of retiring a plan. Trial plans are excluded
+        # too: they are reached through the trial form, which converts them properly.
+        self.fields["plan"].queryset = Plan.objects.filter(is_active=True, is_trial=False)
 
 
 class TrialForm(forms.Form):
-    """Put a club with no subscription yet on a short trial that switches itself to
-    ``post_trial_tier`` automatically once it ends -- see billing.services.dues.start_trial."""
+    """Put a club with no subscription yet on a trial that switches itself to
+    ``post_trial_plan`` automatically once it ends -- see billing.services.dues.start_trial.
 
-    trial_tier = forms.ModelChoiceField(queryset=Tier.objects.none(), label=_("Trial tier"), help_text=_("What this club is billed on during the trial."))
-    post_trial_tier = forms.ModelChoiceField(queryset=Tier.objects.none(), label=_("Then switch to"), help_text=_("The plan it lands on automatically once the trial ends."))
-    trial_months = forms.IntegerField(min_value=1, initial=2, label=_("Trial length (months)"))
+    There is no length field: a trial's length is its plan's own ``duration_months``, so a
+    1-month and a 3-month trial are two plans rather than one plan plus a number typed here.
+    """
+
+    trial_plan = forms.ModelChoiceField(queryset=Plan.objects.none(), label=_("Trial plan"), help_text=_("What this club is billed on during the trial. Its length is the plan's own duration."))
+    post_trial_plan = forms.ModelChoiceField(queryset=Plan.objects.none(), label=_("Then switch to"), help_text=_("The plan it lands on automatically once the trial ends."))
     start = forms.DateField(required=False, widget=forms.DateInput(attrs={"type": "date"}), label=_("Trial starts"), help_text=_("Left blank, the trial starts today."))
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        # Same reasoning as SubscriptionForm: a retired tier keeps billing whoever is
+        # Same reasoning as SubscriptionForm: a retired plan keeps billing whoever is
         # already on it, but must not be offered for a new trial or a new plan either.
-        self.fields["trial_tier"].queryset = Tier.objects.filter(is_active=True)
-        self.fields["post_trial_tier"].queryset = Tier.objects.filter(is_active=True)
+        self.fields["trial_plan"].queryset = Plan.objects.filter(is_active=True, is_trial=True)
+        self.fields["post_trial_plan"].queryset = Plan.objects.filter(is_active=True, is_trial=False)
 
 
 class DuePaymentForm(forms.Form):