Add finance app with Stripe Checkout subscriptions (#21) (#23)

## Summary
- Closes #21 — new Django `finance` app with Stripe as payment provider
- Subscription price defaults to **$10 USD / month** via `SUBSCRIPTION_PRICE_AMOUNT_CENTS = 1000` in `settings.py` (env-overridable)
- Persists **Invoice** and **Payment** rows; both registered in Django admin (with payment inline on invoices)
- Checkout Session API redirects users to Stripe hosted payment; webhook verifies signatures and upserts ledger idempotently

## API
- `POST /api/finance/checkout/` — JWT auth → `{ checkout_url, session_id }`
- `GET /api/finance/invoices/` / `GET /api/finance/payments/` — own records
- `POST /api/finance/webhooks/stripe/` — Stripe signature-verified webhook

## Config
Documented in `.env.example` / `.env.prod.example`:
`STRIPE_SECRET_KEY`, `STRIPE_PUBLISHABLE_KEY`, `STRIPE_WEBHOOK_SECRET`, optional `STRIPE_PRICE_ID`, `FRONTEND_BASE_URL`

## Test plan
- [x] `uv run python manage.py test finance` (17 tests)
- [ ] Set Stripe test keys locally; create checkout session; complete payment in Stripe test mode
- [ ] Confirm Invoice/Payment appear in `/admin/`
- [ ] Point Stripe webhook to `/api/finance/webhooks/stripe/` and verify `checkout.session.completed` / `invoice.paid`Reviewed-on: #23

llm_be/finance/models.py

@@ -0,0 +1,161 @@
+from django.conf import settings
+from django.db import models
+from django.utils import timezone
+
+from chat_backend.models import Company, TimeInfoBase
+
+
+class Invoice(TimeInfoBase):
+    """Local ledger row for a billed period / Stripe invoice or checkout session."""
+
+    class Provider(models.TextChoices):
+        STRIPE = "stripe", "Stripe"
+
+    class Status(models.TextChoices):
+        DRAFT = "draft", "Draft"
+        OPEN = "open", "Open"
+        PAID = "paid", "Paid"
+        VOID = "void", "Void"
+        UNCOLLECTIBLE = "uncollectible", "Uncollectible"
+        PAYMENT_FAILED = "payment_failed", "Payment failed"
+
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="invoices",
+    )
+    company = models.ForeignKey(
+        Company,
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        related_name="invoices",
+    )
+    provider = models.CharField(
+        max_length=32,
+        choices=Provider.choices,
+        default=Provider.STRIPE,
+    )
+    status = models.CharField(
+        max_length=32,
+        choices=Status.choices,
+        default=Status.OPEN,
+        db_index=True,
+    )
+    currency = models.CharField(max_length=8, default="usd")
+    amount_due = models.PositiveIntegerField(
+        default=0,
+        help_text="Amount due in the smallest currency unit (e.g. cents).",
+    )
+    amount_paid = models.PositiveIntegerField(
+        default=0,
+        help_text="Amount paid in the smallest currency unit (e.g. cents).",
+    )
+    period_start = models.DateTimeField(null=True, blank=True)
+    period_end = models.DateTimeField(null=True, blank=True)
+    stripe_invoice_id = models.CharField(
+        max_length=255,
+        blank=True,
+        null=True,
+        unique=True,
+        db_index=True,
+    )
+    stripe_checkout_session_id = models.CharField(
+        max_length=255,
+        blank=True,
+        null=True,
+        unique=True,
+        db_index=True,
+    )
+    stripe_subscription_id = models.CharField(
+        max_length=255,
+        blank=True,
+        null=True,
+        db_index=True,
+    )
+    stripe_customer_id = models.CharField(max_length=255, blank=True, default="")
+    hosted_invoice_url = models.URLField(blank=True, default="")
+    description = models.CharField(max_length=512, blank=True, default="")
+
+    class Meta:
+        ordering = ["-created"]
+
+    def __str__(self) -> str:
+        return f"Invoice {self.pk} ({self.status}) user={self.user_id}"
+
+
+class Payment(TimeInfoBase):
+    """Local ledger row for a payment attempt / Stripe PaymentIntent or charge."""
+
+    class Provider(models.TextChoices):
+        STRIPE = "stripe", "Stripe"
+
+    class Status(models.TextChoices):
+        PENDING = "pending", "Pending"
+        SUCCEEDED = "succeeded", "Succeeded"
+        FAILED = "failed", "Failed"
+        CANCELED = "canceled", "Canceled"
+        REQUIRES_ACTION = "requires_action", "Requires action"
+
+    invoice = models.ForeignKey(
+        Invoice,
+        on_delete=models.CASCADE,
+        related_name="payments",
+        null=True,
+        blank=True,
+    )
+    user = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.CASCADE,
+        related_name="payments",
+    )
+    company = models.ForeignKey(
+        Company,
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        related_name="payments",
+    )
+    provider = models.CharField(
+        max_length=32,
+        choices=Provider.choices,
+        default=Provider.STRIPE,
+    )
+    status = models.CharField(
+        max_length=32,
+        choices=Status.choices,
+        default=Status.PENDING,
+        db_index=True,
+    )
+    currency = models.CharField(max_length=8, default="usd")
+    amount = models.PositiveIntegerField(
+        default=0,
+        help_text="Amount in the smallest currency unit (e.g. cents).",
+    )
+    stripe_payment_intent_id = models.CharField(
+        max_length=255,
+        blank=True,
+        null=True,
+        unique=True,
+        db_index=True,
+    )
+    stripe_charge_id = models.CharField(
+        max_length=255,
+        blank=True,
+        null=True,
+        unique=True,
+        db_index=True,
+    )
+    paid_at = models.DateTimeField(null=True, blank=True)
+    failure_message = models.CharField(max_length=512, blank=True, default="")
+
+    class Meta:
+        ordering = ["-created"]
+
+    def __str__(self) -> str:
+        return f"Payment {self.pk} ({self.status}) user={self.user_id}"
+
+    def mark_succeeded(self, *, paid_at=None):
+        self.status = self.Status.SUCCEEDED
+        self.paid_at = paid_at or timezone.now()
+        self.save(update_fields=["status", "paid_at", "last_modified"])