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.

management/templates/management/_billing_notice.html

@@ -0,0 +1,35 @@
+{% comment %}
+    What this club owes the platform, for its own admins.
+
+    `billing_notice` comes from management.context_processors.billing_notice, which returns
+    None for anyone who is not a club admin -- so this partial never needs to check that
+    itself. Included unconditionally by home.html, and by base.html only when the notice has
+    reached error level, so a final notice follows an admin onto every management page while
+    an early one stays on the dashboard.
+{% endcomment %}
+{% load i18n lucide %}
+
+{% if billing_notice %}
+    <div class="alert {% if billing_notice.level == 'error' %}alert-error{% elif billing_notice.level == 'warning' %}alert-warning{% else %}alert-info{% endif %} mb-6">
+        {% if billing_notice.level == 'error' %}
+            {% lucide "octagon-alert" size=20 %}
+        {% elif billing_notice.level == 'warning' %}
+            {% lucide "triangle-alert" size=20 %}
+        {% else %}
+            {% lucide "receipt-euro" size=20 %}
+        {% endif %}
+        <span>
+            {% blocktrans with amount=billing_notice.amount_outstanding %}Platform fees of €{{ amount }} are outstanding.{% endblocktrans %}
+
+            {% if billing_notice.will_archive %}
+                {% if billing_notice.days_until_archive < 0 %}
+                    {% trans "This club is now due to be archived. Pay to keep access." %}
+                {% else %}
+                    {% blocktrans count days=billing_notice.days_until_archive %}This club will be archived in {{ days }} day unless payment is received.{% plural %}This club will be archived in {{ days }} days unless payment is received.{% endblocktrans %}
+                {% endif %}
+            {% else %}
+                {% trans "Please settle it to keep your account in good standing." %}
+            {% endif %}
+        </span>
+    </div>
+{% endif %}