E2EE – Forward Secrecy (Double Ratchet)

This needs to be implemented to ensure past messages remain secure even if a device is later compromised. The issue should define how message keys evolve per message, how chain states are stored, and how payloads change to support ratcheting. It should also cover migration from the v1 payload format and compatibility handling for older messages. This is part of the overall E2EE initiative.