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  It had been replaced by POST /admin/pricing/setup-stripe but was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.

scripts/stripe_setup.py

@@ -1,77 +0,0 @@
-"""
-Script de setup Stripe : crée les produits et prix dans Stripe Dashboard.
-Lance avec : python scripts/stripe_setup.py
-"""
-import stripe
-import sys
-
-SK = "sk_test_51SkSHkCKXUJE51jnbEtXZ0nKiTHTa8ohDwLH8fZiDVEx6Ze0g5dg4fGJJgX1VgNHvF93GE3HTramT3oQrCaqOxid00OXTcZlsW"
-stripe.api_key = SK
-
-PLANS = [
-    {"id": "starter", "name": "Wordly Starter", "monthly_eur": 900,  "yearly_eur": 8640},
-    {"id": "pro",     "name": "Wordly Pro",     "monthly_eur": 1900, "yearly_eur": 18240},
-    {"id": "business","name": "Wordly Business","monthly_eur": 4900, "yearly_eur": 47040},
-]
-
-results = {}
-for plan in PLANS:
-    print(f"\n>> Creation produit {plan['name']}...")
-    # Search existing product first
-    existing = stripe.Product.search(query=f"name:'{plan['name']}'", limit=1)
-    if existing.data:
-        product = existing.data[0]
-        print(f"  Produit existant: {product.id}")
-    else:
-        product = stripe.Product.create(
-            name=plan["name"],
-            metadata={"plan_id": plan["id"]}
-        )
-        print(f"  Produit créé: {product.id}")
-
-    # Monthly price
-    monthly_prices = stripe.Price.list(product=product.id, active=True, limit=10)
-    monthly_price = None
-    yearly_price = None
-    for p in monthly_prices.data:
-        if p.recurring and p.recurring.interval == "month" and p.unit_amount == plan["monthly_eur"]:
-            monthly_price = p
-        if p.recurring and p.recurring.interval == "year" and p.unit_amount == plan["yearly_eur"]:
-            yearly_price = p
-
-    if not monthly_price:
-        monthly_price = stripe.Price.create(
-            product=product.id,
-            unit_amount=plan["monthly_eur"],
-            currency="eur",
-            recurring={"interval": "month"},
-            metadata={"plan_id": plan["id"], "billing": "monthly"}
-        )
-        print(f"  Prix mensuel créé: {monthly_price.id}")
-    else:
-        print(f"  Prix mensuel existant: {monthly_price.id}")
-
-    if not yearly_price:
-        yearly_price = stripe.Price.create(
-            product=product.id,
-            unit_amount=plan["yearly_eur"],
-            currency="eur",
-            recurring={"interval": "year"},
-            metadata={"plan_id": plan["id"], "billing": "yearly"}
-        )
-        print(f"  Prix annuel créé: {yearly_price.id}")
-    else:
-        print(f"  Prix annuel existant: {yearly_price.id}")
-
-    results[plan["id"]] = {
-        "monthly": monthly_price.id,
-        "yearly": yearly_price.id,
-    }
-
-print("\n" + "="*60)
-print("Variables à ajouter dans .env.production et .env :")
-print("="*60)
-for plan_id, ids in results.items():
-    print(f"STRIPE_PRICE_{plan_id.upper()}_MONTHLY={ids['monthly']}")
-    print(f"STRIPE_PRICE_{plan_id.upper()}_YEARLY={ids['yearly']}")
-print("="*60)