refactor: move vendor info endpoint to tenancy module

MIGRATION:
- Move app/api/v1/vendor/info.py to app/modules/tenancy/routes/api/vendor.py
- Change endpoint path from GET /{vendor_code} to GET /info/{vendor_code}
- Remove catch-all route ordering dependency

TENANCY MODULE SETUP:
- Mark tenancy module as is_self_contained=True
- Add routes/api/vendor.py with vendor_router
- Add exceptions.py with TenancyException hierarchy
- Add placeholder __init__.py files for services, models, schemas

FRONTEND UPDATES:
- Update static/vendor/js/login.js to use new /vendor/info/{vendor_code} path
- Update static/vendor/js/init-alpine.js to use new /vendor/info/{vendor_code} path

The new path is more explicit and eliminates the need for catch-all route
ordering in the vendor router aggregation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/tenancy/models/__init__.py

@@ -0,0 +1,14 @@
+# app/modules/tenancy/models/__init__.py
+"""
+Tenancy module database models.
+
+Models for platform, company, vendor, and admin user management.
+Currently models remain in models/database/ - this package is a placeholder
+for future migration.
+"""
+
+# Models will be migrated here from models/database/
+# For now, import from legacy location if needed:
+# from models.database.vendor import Vendor
+# from models.database.company import Company
+# from models.database.platform import Platform