feat(server): refactor copilot (#14892)

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* **PR #14892** 👈

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packages/backend/server/migrations/20260321124948_copilot_submission_id/migration.sql

@@ -0,0 +1,5 @@
+-- AlterTable
+ALTER TABLE "ai_sessions_messages" ADD COLUMN     "compat_submission_id" VARCHAR;
+
+-- CreateIndex
+CREATE INDEX "ai_sessions_messages_session_id_compat_submission_id_idx" ON "ai_sessions_messages"("session_id", "compat_submission_id");