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/marketing/__init__.py

@@ -0,0 +1,11 @@
+"""Marketing blueprint - landing pages, public content, SEO/GEO assets.
+
+Mounted at root "/" (no url_prefix). Coexists with the legacy /api/* and /app/*
+blueprints. Routes added incrementally in Phase 2 (Tasks A-2.x).
+"""
+from flask import Blueprint
+
+marketing_bp = Blueprint('marketing', __name__)
+
+# Import routes module so it registers route handlers via decorators
+from . import routes  # noqa: E402,F401