feat(marketing): register 3 new Flask blueprints (marketing, billing, legal)

- marketing_bp at root "/"
- billing_bp at /checkout/* (routes added in B-2.7)
- legal_bp at /legal/* (routes added in B-2.9)
- Tests verify all 3 blueprints register correctly
- Coexists with existing recordings_bp at "/" (resolved in B-1.3)

src/billing/__init__.py

@@ -0,0 +1,11 @@
+"""Billing blueprint - Stripe Checkout, webhook, subscription management.
+
+Mounted at /checkout/* prefix for the customer-facing checkout flow. The
+/webhooks/stripe route (added in B-2.8) bypasses the prefix and is also
+csrf-exempted.
+
+Routes added in Tasks B-2.7 (checkout) and B-2.8 (webhook).
+"""
+from flask import Blueprint
+
+billing_bp = Blueprint('billing', __name__, url_prefix='/checkout')