Add integration_type enum to database and registry

Create the migration file using the Supabase CLI: 'supabase migration new add_integration_type_enum'. Use this SQL pattern for safety: first CREATE TYPE IF NOT EXISTS integration_type AS ENUM (...), then ALTER TABLE organization_integrations ALTER COLUMN integration_type TYPE integration_type USING integration_type::integration_type. Add a rollback file with: ALTER TABLE organization_integrations ALTER COLUMN integration_type TYPE text, then DROP TYPE integration_type. The critical cross-stack coupling is between PostgreSQL enum values and Dart enum: add a CI-friendly Dart test that asserts exact value list to catch drift.

Document the process for adding a new integration type in the migration comment (requires: new PG migration to ADD VALUE, new Dart enum value, new registry entry).

Test the migration in a local Supabase development environment. Verify: (1) forward migration runs without errors on empty table; (2) forward migration runs without errors with existing rows containing valid enum values; (3) forward migration fails (or pre-check catches) if rows contain invalid enum values; (4) rollback migration reverts column to text and drops enum without error; (5) after migration, inserting a row with an invalid integration_type value is rejected by PostgreSQL with a type error. Also write a Dart unit test that compares IntegrationTypeRegistry.supportedTypes with a hardcoded list of the 5 expected values — this test acts as a cross-stack sync check.