feat(billing): add BillingService.open_customer_portal + GET endpoint

Authed users can now request a Stripe-hosted Customer Portal URL for card
updates and cancellation via GET /api/v1/billing/portal-session. The path is
already in both _SUBSCRIPTION_GUARD_ALLOWLIST and _EMAIL_VERIFICATION_ALLOWLIST
so canceled or unverified-past-grace users can still update billing.

- Returns 503 with {"error": "stripe_not_configured"} when STRIPE_SECRET_KEY unset.
- Returns 400 with {"error": "no_stripe_customer"} when account has no
  stripe_customer_id (must complete checkout first).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/endpoints/billing.py

@@ -1,6 +1,6 @@
 from typing import Annotated
 
-from fastapi import APIRouter, Depends
+from fastapi import APIRouter, Depends, HTTPException
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -10,6 +10,7 @@ from app.core.config import settings
 from app.models.account import Account
 from app.models.user import User
 from app.schemas.billing import (
+    BillingPortalSessionResponse,
     BillingStateResponse,
     CheckoutSessionCreate,
     CheckoutSessionResponse,
@@ -50,3 +51,26 @@ async def get_billing_state(
     )).scalar_one()
     state = await BillingService.get_billing_state(db, account)
     return BillingStateResponse(**state)
+
+
+@router.get("/portal-session", response_model=BillingPortalSessionResponse)
+async def get_billing_portal_session(
+    current_user: Annotated[User, Depends(get_current_active_user)],
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
+) -> BillingPortalSessionResponse:
+    """Return a Stripe-hosted Customer Portal URL for the account so the user
+    can update card / cancel. Allowlisted from the subscription + email-verify
+    guards (a canceled or unverified-past-grace user must still be able to
+    update billing)."""
+    if not settings.stripe_enabled:
+        raise HTTPException(status_code=503, detail={"error": "stripe_not_configured"})
+
+    account = (await db.execute(
+        select(Account).where(Account.id == current_user.account_id)
+    )).scalar_one()
+
+    try:
+        url = await BillingService.open_customer_portal(account)
+    except ValueError:
+        raise HTTPException(status_code=400, detail={"error": "no_stripe_customer"})
+    return BillingPortalSessionResponse(url=url)