Support offline-mode API mockability for TPEN-Interfaces sync

Enable all (or as many as possible) TPEN-Services API endpoints to be mockable in the context of offline operation. The goal is to allow TPEN-Interfaces to perform actions offline and reliably sync data when reconnected.

Requirements:
- Document which endpoints can be safely mocked and their behaviors in a disconnected scenario
- Provide clear guidance for matching mocked endpoints to real behaviors
- Establish a protocol/interface for syncing up pending changes when connection is restored
- Ensure that API contracts enable roundtrip compatibility (i.e., actions performed offline are compatible with server logic)
- Collaboration with TPEN-Interfaces team to validate offline mock coverage