support: Set discounted price instead percentage for customers.

This allows us to set the price of a plan exactly as discussed with
the customer.

corporate/migrations/0043_remove_customer_default_discount_and_more.py

@@ -0,0 +1,89 @@
+# Generated by Django 5.0.5 on 2024-05-03 06:50
+
+from decimal import Decimal
+
+from django.db import migrations, models
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+# It's generally unsafe to import product code from migrations,
+# because the migration will be run with the **current** version of
+# that code, not the version at the time of the migration. But because
+# this migration will only be run for Zulip Cloud, this is OK.
+from corporate.lib.stripe import get_price_per_license
+
+
+def calculate_discounted_price_per_license(
+    original_price_per_license: int, discount: Decimal
+) -> int:
+    # There are no fractional cents in Stripe, so round down to nearest integer.
+    return int(float(original_price_per_license * (1 - discount / 100)) + 0.00001)
+
+
+def calculate_discounted_price(apps: StateApps, schema_editor: BaseDatabaseSchemaEditor) -> None:
+    """Migrates existing customers with a configured default discount to
+    instead have discounted prices for their configured required_plan_tier.
+
+    Does not operate on CustomerPlan fields, since those are already
+    resolved into a price.
+    """
+    BILLING_SCHEDULE_ANNUAL = 1
+    BILLING_SCHEDULE_MONTHLY = 2
+
+    Customer = apps.get_model("corporate", "Customer")
+    customers_to_update = []
+    for customer in Customer.objects.all():
+        if not customer.required_plan_tier or not customer.default_discount:
+            continue
+
+        annual_price_per_license = get_price_per_license(
+            customer.required_plan_tier, BILLING_SCHEDULE_ANNUAL
+        )
+        customer.annual_discounted_price = calculate_discounted_price_per_license(
+            annual_price_per_license, customer.default_discount
+        )
+        monthly_price_per_license = get_price_per_license(
+            customer.required_plan_tier, BILLING_SCHEDULE_MONTHLY
+        )
+        customer.monthly_discounted_price = calculate_discounted_price_per_license(
+            monthly_price_per_license, customer.default_discount
+        )
+        customers_to_update.append(customer)
+        print(
+            f"\nChanging price for {customer.id}: {customer.default_discount} => {customer.annual_discounted_price}/{customer.monthly_discounted_price}"
+        )
+
+    Customer.objects.bulk_update(
+        customers_to_update, ["annual_discounted_price", "monthly_discounted_price"]
+    )
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("corporate", "0042_invoice_is_created_for_free_trial_upgrade_and_more"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="customer",
+            name="annual_discounted_price",
+            field=models.IntegerField(default=0),
+        ),
+        migrations.AddField(
+            model_name="customer",
+            name="monthly_discounted_price",
+            field=models.IntegerField(default=0),
+        ),
+        # Populate the new discounted price fields based on existing default discount.
+        migrations.RunPython(calculate_discounted_price),
+        migrations.RemoveField(
+            model_name="customer",
+            name="default_discount",
+        ),
+        # This automatically converts the decimal to string so we don't need to do anything here.
+        migrations.AlterField(
+            model_name="customerplan",
+            name="discount",
+            field=models.TextField(null=True),
+        ),
+    ]