feat(chat): add secure dm foundation

[creativaPoeta]

Summary

Add the backend foundation for secure_dm_v1.

Included

• chat security mode and protocol version support
• secure device models and migrations
• public device bundle registration and lookup endpoints
• secure DM text-only message storage and retrieval paths
• per-device encrypted payload storage
• secure message availability socket events
• secure message notifications without plaintext leakage
• support chat explicitly marked as support_plain

Notes

• this is a foundation PR, not the final secure chat rollout
• legacy chat remains intact
• current scope is text-only direct messages
• secure groups, media, reactions, and full ratchet protocol are future phases
• backend foundation was validated with npx tsc --noEmit

Follow-up

• pair with the frontend secure branch
• deploy to a test environment
• run end-to-end secure DM tests before any merge