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)

tests/test_blueprint_registration.py

@@ -39,28 +39,24 @@ def test_legal_blueprint_registered():
 
 
 def test_marketing_landing_route_exists():
-    """The marketing blueprint should expose at least a placeholder root route."""
+    """Marketing blueprint must expose root '/' route."""
     rules = [str(r) for r in app.url_map.iter_rules() if r.endpoint.startswith('marketing.')]
-    assert any('/' in r for r in rules), (
-        f"Expected marketing blueprint to have a route, found: {rules}"
+    assert any(r == '/' for r in rules), (
+        f"Expected marketing root route '/', found: {rules}"
     )
 
 
 def test_legal_blueprint_has_url_prefix():
-    """Legal blueprint should be mounted at /legal/* prefix."""
-    rules = [str(r) for r in app.url_map.iter_rules() if r.endpoint.startswith('legal.')]
-    assert all('/legal' in r for r in rules), (
-        f"Expected /legal/ prefix on all legal routes, found: {rules}"
+    """Legal blueprint must be mounted with /legal url_prefix."""
+    assert 'legal' in app.blueprints
+    assert app.blueprints['legal'].url_prefix == '/legal', (
+        f"Expected legal blueprint url_prefix='/legal', got {app.blueprints['legal'].url_prefix!r}"
     )
 
 
 def test_billing_blueprint_has_url_prefix():
-    """Billing blueprint should be mounted at /checkout/* prefix.
-
-    Phase 1 minimum: blueprint is registered but may have no routes yet.
-    Routes added in B-2.7 (checkout) and B-2.8 (webhook).
-    """
-    rules = [str(r) for r in app.url_map.iter_rules() if r.endpoint.startswith('billing.')]
-    # Allow billing routes that don't start with /checkout (e.g. /webhooks/stripe added later)
-    # but at least placeholder /checkout/<plan> route should exist
-    assert len(rules) >= 0  # Phase 1 minimum: blueprint registered, routes added in B-2.7
+    """Billing blueprint must be mounted with /checkout url_prefix."""
+    assert 'billing' in app.blueprints
+    assert app.blueprints['billing'].url_prefix == '/checkout', (
+        f"Expected billing blueprint url_prefix='/checkout', got {app.blueprints['billing'].url_prefix!r}"
+    )