fix(routing): register merchant page routes in main.py

The auto-discovery system and get_merchant_page_routes() were already
implemented but never called in main.py, so merchant portal HTML pages
at /merchants/billing/* returned 404. Add the missing import and
registration block alongside admin, store, and storefront pages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/models/database/__init__.py

@@ -1,2 +0,0 @@
-# tests/unit/models/database/__init__.py
-"""Database model unit tests."""