[7b] Admin UX validation and error feedback

[andrmaz]

What to build

Harden all admin forms (organizations, departments, sources, policies) to handle invalid inputs with clear, actionable error messages. Verify that non-goal scope constraints (no plugin marketplace, no microservices, no advanced ABAC) are not accidentally introduced. Conduct a final UX pass to ensure the admin experience is coherent enough for a stakeholder demo.

Acceptance criteria

• All admin forms display field-level validation errors on submit.
• Server-side errors (e.g. duplicate name, constraint violation) surface readable messages in the UI.
• A checklist confirms no out-of-scope features have been introduced.
• Manual demo walkthrough of admin flows completes without unexpected errors.

Blocked by

• [6b] Audit log admin viewer #17 [6b] Audit log admin viewer