[kosisochukwu1234]

Summary

Adds docs/adr/0002-signal-dms-mls-groups.md recording the architecture decision to use the Signal Protocol (X3DH + Double Ratchet) for direct messages and Messaging Layer Security (MLS, RFC 9420) for groups.

Details

• Base Branch: Targets dev
• Follows ADR Template: Structured with Context, Decision, Alternatives Rejected, Consequences, and Status.
• Trade-off Analysis: Documents per-device fan-out cost in large groups ($O(N)$ vs. $O(1)$), MLS epoch boundaries and lack of historical messages for new joiners, and library audit status / maturity across runtimes.
• References Existing Material: Cites docs/signal-integration.md and apps/backend/docs/signal-migration.md instead of restating them.
• Transition Plan: Covers the transition from Phase-1 sealed box (never re-encrypting history, envelope protocol tagging, per-device-pair capability negotiation, send-path downgrade rejection).
• Status Section: Accurately reflects what is live in production, what is implemented but not fully wired, and what remains planned.

Closes #642