fix(docker): add PARTY_ROLE environment variable for server-party-api

Add PARTY_ROLE=delegate environment variable to server-party-api service
to fix nil pointer dereference when determining party role during keygen.

Without this variable, the party defaults to "persistent" role which tries
to access keyShareRepo (nil for delegate parties), causing a panic.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/mpc-system/docker-compose.yml

@@ -255,6 +255,7 @@ services:
       MPC_API_KEY: ${MPC_API_KEY}
       # Party identity for Message Router registration
       PARTY_ID: delegate-party
+      PARTY_ROLE: delegate
     depends_on:
       session-coordinator:
         condition: service_healthy