feat(migrate): onboarding backfill (repositories + dast_targets -> onboarded_targets) (#141)

compliance-agent/src/migrate/mod.rs

@@ -0,0 +1,8 @@
+//! One-time data migrations.
+//!
+//! Currently just the onboarding backfill ([`onboarding`]), which folds the
+//! legacy `repositories` and `dast_targets` collections into the unified
+//! `onboarded_targets` collection, preserving `_id` so every downstream record
+//! keyed by `repo_id` / `target_id` keeps resolving.
+
+pub mod onboarding;