Add rollback snapshot controls

AgentCompany-New/src/main/database/migrations/020-rollback-snapshots.ts

@@ -0,0 +1,25 @@
+export const rollbackSnapshotsSchema = `
+  create table if not exists rollback_snapshots (
+    id text primary key,
+    org_id text not null references organizations(id) on delete cascade,
+    migration_report_id text not null references migration_reports(id) on delete cascade,
+    source_database_path text not null,
+    source_fingerprint text not null,
+    source_database_sha256 text not null,
+    workspace_manifest_json text not null,
+    workspace_manifest_sha256 text not null,
+    delta_journal_json text not null,
+    restore_plan_json text not null,
+    active integer not null,
+    created_by_employee_id text references employees(id) on delete set null,
+    created_at text not null,
+    activated_at text
+  );
+
+  create index if not exists idx_rollback_snapshots_org_created on rollback_snapshots(org_id, created_at);
+  create index if not exists idx_rollback_snapshots_report on rollback_snapshots(migration_report_id);
+  create unique index if not exists idx_rollback_snapshots_one_active on rollback_snapshots(org_id) where active = 1;
+
+  insert into schema_migrations(version, name, applied_at)
+  values (20, 'rollback_snapshots', datetime('now'));
+`;