fix(marketing): add template_folder + tighten blueprint registration tests

- Explicit template_folder on marketing/billing/legal blueprints prevents silent
  template fallback in Phase 2
- Replace vacuous test assertions (len>=0, substring '/' in r) with direct
  url_prefix and exact-match route checks (per code review I-1, I-2, I-3)

src/billing/__init__.py

@@ -8,4 +8,10 @@ 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')
+billing_bp = Blueprint(
+    'billing',
+    __name__,
+    url_prefix='/checkout',
+    template_folder='../../templates/billing',
+    static_folder=None,
+)