feat: Feature 3.4 — Stripe integration with founder code system

Subscription billing:
- POST /api/stripe/checkout — creates Stripe Checkout session
- POST /api/stripe/webhook — handles lifecycle events (raw body)
- POST /api/stripe/portal — customer self-service management
- GET /api/stripe/status — current subscription info

Founder code system:
- Validates codes against env var list with expiry date
- Routes to founder Stripe Price IDs (locked rate for life)
- 4 price objects: analyst, analyst-founder, desk, desk-founder

Webhook events handled:
- checkout.session.completed → tier update + founder status
- customer.subscription.deleted → revert to free tier
- invoice.payment_failed → logged

Lazy Stripe init (no API key required at import time).
Raw body middleware for webhook signature verification.

16 new tests, 237 total (210 Node.js + 27 Python), all passing.
Phase 3 Web MVP COMPLETE. All roadmap features shipped.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -30,7 +30,8 @@
     "dotenv": "^17.3.1",
     "express": "^5.2.1",
     "ioredis": "^5.10.1",
-    "postgres": "^3.4.8"
+    "postgres": "^3.4.8",
+    "stripe": "^20.4.1"
   },
   "devDependencies": {
     "jest": "^30.3.0",