stripe: always bill in GBP, never geo-select a currency

/pricing renders "£7" and "£70" as static copy, but checkout sniffed
CF-IPCountry / Accept-Language and passed a matching currency, so Stripe
picked a currency_options rate. A US visitor was shown £7 and charged
$9.99; a German one was charged €7. The page's "Prices in GBP" line was
therefore untrue, and "two months free" only held in GBP and USD (the
EUR annual is €80 against €84, a 4.8% saving).

Displaying one price and billing another is what the UK CPRs and the EU
price-indication rules prohibit, so drop the currency selection rather
than patch the disclosure. Removes _sniff_currency and its country and
locale tables, the currency field on CheckoutRequest, and the now-unused
request parameter.

The currency_options configured on the live Prices are left in place but
unused. Reinstating geo-pricing requires making /pricing currency-aware
first — copy, buttons and the currency-specific saving claim.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/routers/stripe_billing.py

@@ -19,7 +19,7 @@ from __future__ import annotations
 
 import asyncio
 import json
-from typing import Any, Literal, Optional
+from typing import Any, Literal
 
 import stripe
 from fastapi import APIRouter, Body, Depends, HTTPException, Request
@@ -75,51 +75,21 @@ def _price_for(cadence: str) -> str:
     raise HTTPException(status_code=400, detail="cadence must be 'monthly' or 'annual'")
 
 
-# Rough country → currency mapping. Covers the markets we have a stated
-# rate for; everything else falls back to GBP (the home currency) and
-# Stripe handles the FX at checkout. Configure the per-currency
-# unit_amount on each Price's `currency_options` in the Stripe Dashboard
-# — we just signal which option to use here.
-_COUNTRY_CURRENCY: dict[str, str] = {
-    "US": "usd", "CA": "usd",
-    "GB": "gbp", "IM": "gbp", "JE": "gbp", "GG": "gbp",
-    **dict.fromkeys((
-        "DE", "FR", "IT", "ES", "PT", "NL", "BE", "IE", "AT", "FI",
-        "GR", "LU", "MT", "CY", "EE", "LV", "LT", "SI", "SK", "HR",
-    ), "eur"),
-}
-
-# Accept-Language locale → currency, used when CF-IPCountry is absent.
-# Ambiguous locales (e.g. plain "fr" without region) get EUR because
-# that's the majority outcome.
-_LOCALE_CURRENCY: dict[str, str] = {
-    "en-gb": "gbp", "en": "gbp",
-    "en-us": "usd", "en-ca": "usd",
-    "fr": "eur", "de": "eur", "it": "eur", "es": "eur",
-    "pt": "eur", "nl": "eur",
-}
-
-
-def _sniff_currency(request: Request) -> str:
-    """Best-effort currency detection for new-customer checkouts.
-
-    Order: explicit Cloudflare country header, then Accept-Language
-    (exact match then language-only). GBP as the final fallback. Only
-    consulted when the user has no Stripe customer record yet — Stripe
-    locks currency at customer creation, so an existing customer's
-    currency wins regardless of the request locale.
-    """
-    cc = (request.headers.get("cf-ipcountry") or "").upper()
-    if cc in _COUNTRY_CURRENCY:
-        return _COUNTRY_CURRENCY[cc]
-    al = (request.headers.get("accept-language") or "").lower()
-    first = al.split(",", 1)[0].split(";", 1)[0].strip()
-    if first in _LOCALE_CURRENCY:
-        return _LOCALE_CURRENCY[first]
-    short = first.split("-", 1)[0]
-    if short in _LOCALE_CURRENCY:
-        return _LOCALE_CURRENCY[short]
-    return "gbp"
+# NOTE: we deliberately never pass `currency` to Stripe, so every
+# checkout bills the Price's base currency — GBP. An earlier version
+# sniffed CF-IPCountry / Accept-Language and selected a matching
+# `currency_options` entry, but /pricing renders £7 and £70 as static
+# copy: a US visitor was shown £7 and charged $9.99. Showing one price
+# and billing another is exactly what the UK CPRs and the EU
+# price-indication rules prohibit, so the sniffing was removed rather
+# than the disclosure patched. The `currency_options` still configured
+# on the Prices in the Dashboard are simply unused.
+#
+# To reinstate geo-pricing, /pricing must render the matching currency
+# in its copy, its buttons AND its annual-saving claim first (the claim
+# is currency-specific: "two months free" is true at £70/£84 and
+# $94.99/$119.88, but not at €80/€84). See git history for the removed
+# _sniff_currency helper and its country/locale tables.
 
 
 def _stripe_client() -> stripe.StripeClient:
@@ -136,10 +106,6 @@ def _stripe_client() -> stripe.StripeClient:
 
 class CheckoutRequest(BaseModel):
     cadence: Literal["monthly", "annual"]
-    # Optional override; when omitted we sniff from request headers.
-    # Honoured only for first-time checkouts (Stripe locks currency
-    # to the customer at creation).
-    currency: Optional[Literal["gbp", "usd", "eur"]] = None
 
 
 class CheckoutResponse(BaseModel):
@@ -149,7 +115,6 @@ class CheckoutResponse(BaseModel):
 @router.post("/api/stripe/checkout", response_model=CheckoutResponse)
 async def create_checkout(
     body: CheckoutRequest,
-    request: Request,
     session: AsyncSession = Depends(get_session),
     cu: CurrentUser = Depends(require_auth),
 ) -> CheckoutResponse:
@@ -178,13 +143,10 @@ async def create_checkout(
         # referral redemption flow ships.
         "allow_promotion_codes": True,
     }
-    # Multi-currency: for first-time buyers (no stripe_customer_id yet)
-    # we pass the detected/requested currency. Stripe picks the matching
-    # `currency_options` rate configured on the Price in the Dashboard,
-    # then locks that currency to the new customer record. Existing
-    # customers keep their original currency regardless.
-    if not user.stripe_customer_id:
-        create_kwargs["currency"] = body.currency or _sniff_currency(request)
+    # No `currency` kwarg — every checkout bills the Price's base
+    # currency (GBP), matching the static £7 / £70 copy on /pricing.
+    # See the note above _stripe_client() before reintroducing one.
+    #
     # Per-cadence cooling-off treatment:
     #
     # - Annual gets a 14-day free trial. No money moves during the