feat(migration): add keygen_session_id column to mpc_sessions table

For sign sessions, this column stores the reference to the keygen session
whose key shares should be used for signing.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/migrations/007_add_keygen_session_id_to_mpc_sessions.down.sql

@@ -0,0 +1,7 @@
+-- Migration 007 Down: Remove keygen_session_id from mpc_sessions table
+
+-- Drop index
+DROP INDEX IF EXISTS idx_mpc_sessions_keygen_session_id;
+
+-- Remove column
+ALTER TABLE mpc_sessions DROP COLUMN IF EXISTS keygen_session_id;

backend/mpc-system/migrations/007_add_keygen_session_id_to_mpc_sessions.up.sql

@@ -0,0 +1,12 @@
+-- Migration 007: Add keygen_session_id to mpc_sessions table
+-- Description: For sign sessions, this column stores the reference to the keygen session
+--              whose key shares should be used for signing.
+
+-- Add keygen_session_id column to mpc_sessions table
+ALTER TABLE mpc_sessions ADD COLUMN keygen_session_id UUID;
+
+-- Add index for efficient lookups
+CREATE INDEX idx_mpc_sessions_keygen_session_id ON mpc_sessions(keygen_session_id);
+
+-- Add comment
+COMMENT ON COLUMN mpc_sessions.keygen_session_id IS 'For sign sessions: references the keygen session whose key shares to use';