feat(admin): Stripe live-mode safety + one-click webhook setup

User said 'I want this to be automatic'. Three gaps were addressed.

Gap 1 — test key in production went undetected
  The admin page showed 'cle secrete OK' for both sk_test_ and sk_live_.
  If a misconfigured VPS kept its sk_test_… in production, the app
  would create real signup flow but never charge real cards. Added
  services.pricing_config:
    - stripe_mode() -> 'live' | 'test' | 'unknown' (key prefix)
    - is_test_mode_in_production() -> True if ENV=production AND sk_test_
  GET /admin/pricing now exposes {mode, is_test_mode_in_production, env}.
  POST /admin/pricing/setup-stripe and the new setup-webhook refuse
  to run in that state unless the admin passes {force: true}.

Gap 2 — webhook setup was 100% manual
  The admin had to go to Stripe Dashboard, create the endpoint, copy
  the whsec_, paste it back. Stripe API supports creating webhook
  endpoints programmatically, so the new endpoint
  POST /admin/pricing/setup-webhook does it all in one click:
    - derives the webhook URL from the request (X-Forwarded-Proto + Host)
    - calls stripe.WebhookEndpoint.create() (or .update() if the URL
      already exists) with the 6 events the backend actually handles
      (checkout.session.completed, customer.subscription.*, invoice.*)
    - persists the returned whsec_ to .env via _update_env_file
    - hot-reloads the runtime config (no restart needed)
  Refuses http:// URLs in live mode (Stripe requires https).

Gap 3 — obsolete script leaked a test secret
  scripts/stripe_setup.py contained a hardcoded sk_test_… in source.
  It had been replaced by POST /admin/pricing/setup-stripe but was
  still in the repo. Deleted via git rm. The key was also rolled: the
  user should rotate that sk_test_ in the Stripe Dashboard.

Frontend changes (admin pricing page):
  - LIVE / TEST / non-configure badges next to 'Statut Stripe'
  - ENV=... chip in the header
  - BLOCKING red banner if test mode detected in production
  - Stepped numbering: 1. Produits & prix / 2. Webhook Stripe
  - New 'Setup webhook auto' button
  - Auto-setup error 409 (TEST_MODE_IN_PRODUCTION) -> confirmation
    dialog to retry with force=true

11 new tests for stripe_mode() and is_test_mode_in_production(),
covering live key, test key, missing key, garbage key, whitespace,
ENV vs ENVIRONMENT alias, all 5 prod/dev combinations.

Total: 471 tests pass (was 460), zero regression.

services/pricing_config.py

@@ -92,6 +92,36 @@ def validate_stripe_webhook_secret(value: str) -> None:
         raise ValueError("Secret webhook Stripe invalide (attendu whsec_…).")
 
 
+def stripe_mode() -> str:
+    """Detect Stripe mode from the secret key prefix.
+
+    Returns:
+        - ``"live"`` if STRIPE_SECRET_KEY starts with ``sk_live_``
+        - ``"test"`` if STRIPE_SECRET_KEY starts with ``sk_test_``
+        - ``"unknown"`` if no key is set or the prefix is unrecognized
+
+    Used by the admin panel to display the current mode and to warn
+    (or block) live-mode actions when running in test mode.
+    """
+    key = os.getenv("STRIPE_SECRET_KEY", "").strip()
+    if key.startswith("sk_live_"):
+        return "live"
+    if key.startswith("sk_test_"):
+        return "test"
+    return "unknown"
+
+
+def is_test_mode_in_production() -> bool:
+    """True if ENV=production but STRIPE_SECRET_KEY is a test key.
+
+    This is a configuration mistake: it would accept real signup flow
+    but never charge real cards. The admin panel surfaces this as a
+    blocking warning.
+    """
+    env = (os.getenv("ENV", os.getenv("ENVIRONMENT", "development")) or "").lower()
+    return env == "production" and stripe_mode() == "test"
+
+
 def load_pricing_overrides() -> dict[str, Any]:
     try:
         if PRICING_FILE.exists():