fix(core): ensure correct tenant isolation and fallback behavior for managers and mixins

[CFBruna]

What Changed

• Corrected TenantManager.get_queryset() to fallback to the base queryset instead of qs.none(), fixing breakages outside of request contexts (e.g., Celery, Admin).
• Updated ClinicScopeMixin to correctly honor the get_active_clinic context, including overriding is_staff when a clinic header is provided, while maintaining cross-clinic access for floating employees.
• Fixed string evaluation inside an f-string using str() for gettext_lazy translations in financial_service.py.
• Added new regression tests with 8 distinct isolation/bypass test scenarios to test_tenant_middleware.py.
• Replaced TenantManager.for_clinic() chain filtering with a direct super().get_queryset() call, preventing empty results when querying across clinics.
• Renamed and initialized tests directories replacing standalone tests.py conflicts.

Why

Phase 0 Foundation Hardening requires the DB scope filtering to be solid. Returning qs.none() blindly prevented automated background jobs and command lines to operate correctly outside the HTTP flow. At the same time, Admin boundaries must still be strictly functional inside HTTP, so that providing an X-Clinic-ID accurately simulates clinic scopes exactly without hard-rejections.

How to Test

• Authenticate via make start as an Admin user.
• Add products pointing to Clinic 1 and Clinic 2.
• Bypass tests simulating a superuser query through Postman/Curl: curl -s -H "Authorization: Token <TOKEN>" -H "X-Clinic-ID: 2" http://localhost:8000/api/v1/store/products/
• Run cd backend && uv run pytest ensuring tests yield coverage for all 8 scenarios cleanly within src/apps/core/tests/test_tenant_middleware.py.
• Utilize for_clinic manager in a terminal DB inspection seeing it retrieve correctly specific clinics without crashing.

Checklist

• Service layer architecture followed
• No ORM in views
• No hardcoded strings (frontend and backend)
• Translation keys added to en, pt, es
• Tests added or updated
• uv run pytest passes
• npm run build passes
• Turing Test passed (works with PYG .env)