refactor(tests): reorganize tests per module with shared root conftest

Move 42 single-module test files into app/modules/*/tests/ directories
while keeping 40 cross-module and infrastructure tests central in tests/.
Hub fixtures (engine, db, client, cleanup) moved to root conftest.py so
both tests/ and module tests inherit them. Update pyproject.toml testpaths
and Makefile TEST_PATHS to discover all test locations.

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

Makefile

@@ -250,8 +250,8 @@ ifdef frontend
     endif
 endif
 
-# All testpaths
-TEST_PATHS := tests/
+# All testpaths (central + module tests)
+TEST_PATHS := tests/ app/modules/tenancy/tests/ app/modules/catalog/tests/ app/modules/billing/tests/ app/modules/messaging/tests/ app/modules/orders/tests/ app/modules/customers/tests/ app/modules/marketplace/tests/ app/modules/inventory/tests/ app/modules/loyalty/tests/
 
 test:
 	@docker compose -f docker-compose.test.yml up -d 2>/dev/null || true