Per-user acknowledgement tracking and policy artifact file restrictions

User-level policy acknowledgement tracking. Iru Compliance now tracks each user's policy acknowledgements individually, giving admins a durable, per-user record of every accept and re-accept for every policy.

Only PDF and DOCX allowed for policy artifacts. Policy artifact uploads are now restricted to PDF and DOCX file types to keep the policy library clean and ensure inline preview works reliably for every published policy.