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

src/lib/stripe/stripe-products.ts

@@ -0,0 +1,48 @@
+/**
+ * Stripe product / price id 映射
+ *
+ * 为什么不直接存到 `subscription-plans.ts`：
+ *   - product/price id 是部署环境配置（不同环境不同 product）
+ *   - 套餐本身的 `name` / `price` / `highlight` 是业务常量（不随环境变）
+ *   - 分离后，套餐表只包含业务字段，产品 id 走 env —— 避免硬编码 + 生产安全
+ *
+ * env 命名规范：
+ *   - STRIPE_PRODUCT_<PLAN_ID_UPPER> → Product ID
+ *   - STRIPE_PRICE_<PLAN_ID_UPPER>   → Price ID
+ *   - 例：STRIPE_PRODUCT_MONTHLY, STRIPE_PRICE_QUARTERLY
+ */
+import type { SubscriptionPlanId } from "@/data/constants/subscription-plans";
+
+/**
+ * 获取 Stripe Product ID（从 env）
+ *
+ * @throws Error 如果 env 未配置（避免 silent failure）
+ */
+export function getPlanProductId(planId: SubscriptionPlanId): string {
+  const envKey = `STRIPE_PRODUCT_${planId.toUpperCase()}`;
+  const productId = process.env[envKey];
+  if (!productId || productId.startsWith("prod_replace_")) {
+    throw new Error(
+      `Stripe product id is not configured. Set ${envKey} in .env.local.`,
+    );
+  }
+  return productId;
+}
+
+/**
+ * 获取 Stripe Price ID（从 env）
+ *
+ * Price ID 是真正用在 `stripe.checkout.sessions.create({ line_items: [{ price: "price_xxx" }] })` 里的
+ *
+ * @throws Error 如果 env 未配置
+ */
+export function getPlanPriceId(planId: SubscriptionPlanId): string {
+  const envKey = `STRIPE_PRICE_${planId.toUpperCase()}`;
+  const priceId = process.env[envKey];
+  if (!priceId || priceId.startsWith("price_replace_")) {
+    throw new Error(
+      `Stripe price id is not configured. Set ${envKey} in .env.local.`,
+    );
+  }
+  return priceId;
+}