chore: remove dead Supabase/Stripe env vars from docker-compose and env example

Frontend build args and runtime env no longer need NEXT_PUBLIC_SUPABASE_URL,
NEXT_PUBLIC_SUPABASE_ANON_KEY, SUPABASE_SERVICE_ROLE_KEY, STRIPE_SECRET_KEY,
STRIPE_WEBHOOK_SECRET — all replaced by V2 gateway. .env.v2.example updated to
reflect the current V2-only config.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.v2.example

@@ -26,6 +26,12 @@ CORS_ORIGIN=http://localhost:3000
 # The only backend port exposed to the host. Change if 8080 is taken locally.
 GATEWAY_PORT=8080
 
+# ── Frontend public vars (baked into the Next.js image at build time) ────────
+NEXT_PUBLIC_SITE_URL=http://localhost:3000
+# Browser-facing gateway URL (host port). Must match GATEWAY_PORT above.
+NEXT_PUBLIC_API_URL=http://localhost:8080/v1
+NEXT_PUBLIC_TENANT_SLUG=flatrender
+
 # ── ZarinPal (Iranian payment gateway) ───────────────────────────────────────
 # Get your merchant ID from https://www.zarinpal.com/
 ZARINPAL_MERCHANT_ID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
@@ -49,12 +55,4 @@ TARA_CALLBACK_URL=https://yourdomain.com/v1/payments/callback/tara
 # ── Stripe (international payment gateway) ───────────────────────────────────
 # Get keys from https://dashboard.stripe.com/apikeys
 STRIPE_SECRET_KEY=sk_test_...
-STRIPE_WEBHOOK_SECRET=whsec_...
 STRIPE_PUBLISHABLE_KEY=pk_test_...
-
-# ── Next.js frontend (NEXT_PUBLIC_* baked at build time) ─────────────────────
-NEXT_PUBLIC_SUPABASE_URL=https://your-project.supabase.co
-NEXT_PUBLIC_SUPABASE_ANON_KEY=eyJ...
-NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=pk_test_...
-NEXT_PUBLIC_SITE_URL=http://localhost:3000
-SUPABASE_SERVICE_ROLE_KEY=eyJ...