feat: integrate Stripe payment with subscription plans

Add Stripe payment integration across the project:

- Add @stripe/stripe-js dependency
- Configure Stripe environment variables (secret key, publishable key, webhook secret) for dev, local, and production environments
- Add Product/Price IDs for monthly, quarterly, and annual subscription tiers
- Extend subscription plan data with voiceMinutesPerDay quotas (30/45/60 minutes per tier)
- Update .gitignore to exclude implementation_plan.md

env-example/.env.production.example

@@ -29,3 +29,20 @@ AUTH_FACEBOOK_SECRET=004b1a9384b3433e153992e5edcef4ad
 # Cloudflare CDN（test / production 部署后刷缓存用）
 CF_ZONE_ID=b1b6bfb7795667609c8e7f418af0156b
 CF_API_TOKEN=cfut_xyHb70XCw0j8f5Wz5fg5PemUQ2ZHnSpWvmrBvVeK94d3b00f
+
+# ─────────────────────────────
+# Stripe 支付（**生**产**环**境** live keys —— **从** https://dashboard.stripe.com/apikeys 拿**到**）
+# **不**要** commit 真**实** key 到**版**本**控**制**
+# ─────────────────────────────
+STRIPE_SECRET_KEY=sk_live_replace_with_your_live_secret_key
+NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=pk_live_replace_with_your_live_publishable_key
+# webhook 签**名**密**钥**（**从** Stripe dashboard / Webhooks **端**点**的** "Signing secret" **拿**到**）
+STRIPE_WEBHOOK_SECRET=whsec_replace_with_your_live_webhook_signing_secret
+
+STRIPE_PRODUCT_MONTHLY=prod_replace_monthly
+STRIPE_PRICE_MONTHLY=price_replace_monthly
+STRIPE_PRODUCT_QUARTERLY=prod_replace_quarterly
+STRIPE_PRICE_QUARTERLY=price_replace_quarterly
+STRIPE_PRODUCT_ANNUAL=prod_replace_annual
+STRIPE_PRICE_ANNUAL=price_replace_annual
+