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,
+)

src/legal/__init__.py

@@ -8,4 +8,10 @@ Routes added in Task B-2.9.
 """
 from flask import Blueprint
 
-legal_bp = Blueprint('legal', __name__, url_prefix='/legal')
+legal_bp = Blueprint(
+    'legal',
+    __name__,
+    url_prefix='/legal',
+    template_folder='../../templates/legal',
+    static_folder=None,
+)

src/marketing/__init__.py

@@ -5,7 +5,12 @@ blueprints. Routes added incrementally in Phase 2 (Tasks A-2.x).
 """
 from flask import Blueprint
 
-marketing_bp = Blueprint('marketing', __name__)
+marketing_bp = Blueprint(
+    'marketing',
+    __name__,
+    template_folder='../../templates/marketing',
+    static_folder=None,
+)
 
 # Import routes module so it registers route handlers via decorators
 from . import routes  # noqa: E402,F401