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/data/dto/user/user.ts

@@ -24,6 +24,14 @@ export class User {
   declare readonly avatarUrl: string;
   declare readonly loginProvider: string;
   declare readonly isGuest: boolean;
+  /** 是否为 VIP 会员（Stripe webhook 更新） */
+  declare readonly isVip: boolean;
+  /** 语音聊天剩余分钟数（Stripe webhook 更新） */
+  declare readonly voiceMinutesRemaining: number;
+  /** Stripe Customer ID —— 首次 Checkout 完成后由 webhook 设置 */
+  declare readonly stripeCustomerId: string | null;
+
+
 
   private constructor(input: UserInput) {
     const data = UserSchema.parse(input);