feat: Add Stripe payment integration and subscription management

- Add Stripe.net SDK for payment processing
- Implement StripeService with checkout sessions, customer portal, webhooks
- Implement SubscriptionService for quota management
- Add quota enforcement to CVCheckService
- Create Pricing, Billing, Settings pages
- Add checkout success/cancel pages
- Update Check and Dashboard with usage indicators
- Add ResetMonthlyUsageJob for billing cycle resets
- Add database migration for subscription fields

Plan tiers: Free (3 checks), Professional £49/mo (30), Enterprise £199/mo (unlimited)

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

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

src/TrueCV.Web/appsettings.json

@@ -10,6 +10,15 @@
   "Anthropic": {
     "ApiKey": ""
   },
+  "Stripe": {
+    "SecretKey": "",
+    "PublishableKey": "",
+    "WebhookSecret": "",
+    "PriceIds": {
+      "Professional": "",
+      "Enterprise": ""
+    }
+  },
   "DefaultAdmin": {
     "Email": "",
     "Password": ""