docs: add Stripe setup guide with webhook secret instructions

Add step-by-step setup guide covering:
- Getting API keys from Stripe Dashboard
- Creating webhook endpoint and getting signing secret
- Local development with Stripe CLI
- Creating products and prices

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/features/subscription-billing.md

@@ -169,9 +169,60 @@ Required environment variables:
 STRIPE_SECRET_KEY=sk_test_...
 STRIPE_PUBLISHABLE_KEY=pk_test_...
 STRIPE_WEBHOOK_SECRET=whsec_...
-STRIPE_TRIAL_DAYS=14  # Optional, default trial period
+STRIPE_TRIAL_DAYS=30  # Optional, default trial period
 ```
 
+### Setup Guide
+
+#### Step 1: Get API Keys
+
+1. Go to [Stripe Dashboard](https://dashboard.stripe.com/apikeys)
+2. Copy your **Publishable key** (`pk_test_...` or `pk_live_...`)
+3. Copy your **Secret key** (`sk_test_...` or `sk_live_...`)
+
+#### Step 2: Create Webhook Endpoint
+
+1. Go to [Stripe Webhooks](https://dashboard.stripe.com/webhooks)
+2. Click **Add endpoint**
+3. Enter your endpoint URL: `https://yourdomain.com/api/v1/webhooks/stripe`
+4. Select events to listen to:
+   - `checkout.session.completed`
+   - `customer.subscription.created`
+   - `customer.subscription.updated`
+   - `customer.subscription.deleted`
+   - `invoice.paid`
+   - `invoice.payment_failed`
+5. Click **Add endpoint**
+6. Copy the **Signing secret** (`whsec_...`) - this is your `STRIPE_WEBHOOK_SECRET`
+
+#### Step 3: Local Development with Stripe CLI
+
+For local testing, use the [Stripe CLI](https://stripe.com/docs/stripe-cli):
+
+```bash
+# Install Stripe CLI
+brew install stripe/stripe-cli/stripe  # macOS
+# or download from https://github.com/stripe/stripe-cli/releases
+
+# Login to Stripe
+stripe login
+
+# Forward webhooks to your local server
+stripe listen --forward-to localhost:8000/api/v1/webhooks/stripe
+
+# The CLI will display a webhook signing secret (whsec_...)
+# Use this as STRIPE_WEBHOOK_SECRET for local development
+```
+
+#### Step 4: Create Products & Prices in Stripe
+
+Create subscription products for each tier:
+
+1. Go to [Stripe Products](https://dashboard.stripe.com/products)
+2. Create products for each tier (Starter, Professional, Business, Enterprise)
+3. Add monthly and annual prices for each
+4. Copy the Price IDs (`price_...`) and update your tier configuration
+
 ### Webhook Events
 
 The system handles these Stripe events: