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/stores/user/user-machine.ts

@@ -50,6 +50,8 @@ function toView(u: {
   currentMood?: string;
   isGuest?: boolean;
   isVip?: boolean;
+  voiceMinutesRemaining?: number;
+  stripeCustomerId?: string | null;
 }): UserView {
   return {
     id: u.id,
@@ -62,6 +64,8 @@ function toView(u: {
     currentMood: u.currentMood ?? "",
     isGuest: u.isGuest ?? false,
     isVip: u.isVip ?? false,
+    voiceMinutesRemaining: u.voiceMinutesRemaining ?? 0,
+    stripeCustomerId: u.stripeCustomerId ?? null,
   };
 }