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>

main.py

@@ -64,6 +64,7 @@ from app.exceptions.handler import setup_exception_handlers
 # Module route auto-discovery - all page routes now come from modules
 from app.modules.routes import (
     get_admin_page_routes,
+    get_merchant_page_routes,
     get_platform_page_routes,
     get_storefront_page_routes,
     get_store_page_routes,
@@ -358,6 +359,23 @@ for route_info in admin_page_routes:
         include_in_schema=route_info.include_in_schema,
     )
 
+# =============================================================================
+# MERCHANT PAGES (Merchant billing portal)
+# =============================================================================
+logger.info("Auto-discovering merchant page routes...")
+merchant_page_routes = get_merchant_page_routes()
+logger.info(f"  Found {len(merchant_page_routes)} merchant page route modules")
+
+for route_info in merchant_page_routes:
+    prefix = f"/merchants{route_info.custom_prefix}" if route_info.custom_prefix else "/merchants"
+    logger.info(f"  Registering {route_info.module_code} merchant pages at {prefix}")
+    app.include_router(
+        route_info.router,
+        prefix=prefix,
+        tags=route_info.tags,
+        include_in_schema=route_info.include_in_schema,
+    )
+
 # =============================================================================
 # STORE PAGES
 # =============================================================================

tests/unit/models/database/__init__.py

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