Ensure 100 E2E tests and fix integration catalog bugs

[google-labs-jules]

This PR ensures that there are at least 100 end-to-end integration tests covering the full stack. It also fixes bugs found during testing:

1. The Integration Catalog endpoint was returning 500 errors because the database seed data for triggers and actions was in the wrong format (strings instead of dicts). This was fixed by creating a robust fallback seeder backend/scripts/seed_integrations_fallback.py.
2. The User Registration endpoint was accepting invalid email addresses. This was fixed by enforcing EmailStr validation in backend/core/auth_endpoints.py.
3. The test suite in tests/e2e/full_stack_coverage.spec.ts was expanded to 100 tests to meet the requirement.

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PR created automatically by Jules for task 8689437116413832293 started by @rush86999

[google-labs-jules]

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