Admin Stripe invoices + recurring subscriptions (customers pay us) (#24)

## Summary
- Closes #23
- Admin-only invoice dashboard under `/financial/invoices` (superuser)
- One-off Stripe hosted invoices + email pay link (customers pay us)
- Monthly (or weekly/yearly) Stripe Checkout subscriptions + email checkout link
- Webhook at `/financial/stripe/webhook/` syncs invoice/subscription status
- Env docs: `STRIPE_SECRET_KEY`, `STRIPE_PUBLISHABLE_KEY`, `STRIPE_WEBHOOK_SECRET`

## Test plan
- [ ] Set Stripe **test** keys in local `.env` and restart app
- [ ] As superuser: Dashboard → Invoices → create one-off invoice; confirm Stripe invoice + email
- [ ] As superuser: create monthly subscription; open Checkout link; complete with test card `4242…`
- [ ] Configure Stripe CLI or Dashboard webhook → confirm status updates to paid/active
- [ ] Non-admin user cannot open `/financial/invoices`
- [ ] Prod: add live keys + webhook secret to server `.env`, migrate, restart; point webhook to `https://aimloperations.com/financial/stripe/webhook/`

## Notes
Uses stdlib HTTP to Stripe API (no `stripe` PyPI package) so deploys stay dependency-light.Reviewed-on: #24

company_site/financial/models.py

@@ -302,6 +302,108 @@ class TimeCardCell(IdMixin, TimeMixin):
     charge_number = models.ForeignKey(ChargeNumber, on_delete=models.CASCADE, null=True, blank=True)
 
 
+class BillingCustomer(IdMixin, TimeMixin):
+    """Customer who pays us (one-off invoices or subscriptions)."""
+
+    name = models.CharField(max_length=200)
+    email = models.EmailField()
+    company = models.CharField(max_length=200, blank=True)
+    stripe_customer_id = models.CharField(max_length=255, blank=True, default="")
+    notes = models.TextField(blank=True)
+
+    class Meta:
+        ordering = ["name"]
+
+    def __str__(self):
+        return f"{self.name} <{self.email}>"
+
+
+class Invoice(IdMixin, TimeMixin):
+    """One-off invoice — customer pays us via Stripe hosted invoice URL."""
+
+    class Status(models.TextChoices):
+        DRAFT = "draft", "Draft"
+        OPEN = "open", "Open"
+        PAID = "paid", "Paid"
+        VOID = "void", "Void"
+        UNCOLLECTIBLE = "uncollectible", "Uncollectible"
+        FAILED = "failed", "Failed"
+
+    customer = models.ForeignKey(
+        BillingCustomer, on_delete=models.PROTECT, related_name="invoices"
+    )
+    description = models.CharField(max_length=500)
+    amount_cents = models.PositiveIntegerField(help_text="Amount in cents (USD)")
+    currency = models.CharField(max_length=3, default="usd")
+    status = models.CharField(
+        max_length=20, choices=Status.choices, default=Status.DRAFT
+    )
+    due_date = models.DateField(null=True, blank=True)
+    notes = models.TextField(blank=True)
+    stripe_invoice_id = models.CharField(max_length=255, blank=True, default="")
+    hosted_invoice_url = models.URLField(max_length=500, blank=True, default="")
+    invoice_pdf_url = models.URLField(max_length=500, blank=True, default="")
+    pay_link_emailed_at = models.DateTimeField(null=True, blank=True)
+
+    class Meta:
+        ordering = ["-created"]
+
+    def __str__(self):
+        return f"Invoice {self.pk} — {self.customer} — {self.status}"
+
+    @property
+    def amount_dollars(self):
+        return self.amount_cents / 100.0
+
+
+class RecurringSubscription(IdMixin, TimeMixin):
+    """Monthly (or custom) recurring charge — customer pays us via Stripe Subscription."""
+
+    class Status(models.TextChoices):
+        INCOMPLETE = "incomplete", "Incomplete"
+        ACTIVE = "active", "Active"
+        PAST_DUE = "past_due", "Past due"
+        CANCELED = "canceled", "Canceled"
+        UNPAID = "unpaid", "Unpaid"
+        TRIALING = "trialing", "Trialing"
+        PAUSED = "paused", "Paused"
+
+    class Interval(models.TextChoices):
+        MONTH = "month", "Monthly"
+        YEAR = "year", "Yearly"
+        WEEK = "week", "Weekly"
+
+    customer = models.ForeignKey(
+        BillingCustomer, on_delete=models.PROTECT, related_name="subscriptions"
+    )
+    description = models.CharField(max_length=500)
+    amount_cents = models.PositiveIntegerField(help_text="Amount per period in cents (USD)")
+    currency = models.CharField(max_length=3, default="usd")
+    interval = models.CharField(
+        max_length=10, choices=Interval.choices, default=Interval.MONTH
+    )
+    status = models.CharField(
+        max_length=20, choices=Status.choices, default=Status.INCOMPLETE
+    )
+    notes = models.TextField(blank=True)
+    stripe_product_id = models.CharField(max_length=255, blank=True, default="")
+    stripe_price_id = models.CharField(max_length=255, blank=True, default="")
+    stripe_subscription_id = models.CharField(max_length=255, blank=True, default="")
+    checkout_session_id = models.CharField(max_length=255, blank=True, default="")
+    checkout_url = models.URLField(max_length=500, blank=True, default="")
+    pay_link_emailed_at = models.DateTimeField(null=True, blank=True)
+
+    class Meta:
+        ordering = ["-created"]
+
+    def __str__(self):
+        return f"Subscription {self.pk} — {self.customer} — {self.status}"
+
+    @property
+    def amount_dollars(self):
+        return self.amount_cents / 100.0
+
+
 def set_user_type(user, user_type):
     """Set user type and sync the Employee record (mutually exclusive types)."""
     user.__dict__.pop("profile", None)