docs(m003): add dormant-columns rationale comment (review T1 minor #1)

src/main/db/migrations/m003_soft_delete.ts

@@ -1,3 +1,6 @@
+// v3: soft delete (#4) introduces deleted_at.
+// last_recalled_at + recall_dismissed_at are pre-allocated for #6 (recall) —
+// dormant until then to avoid a v4 migration round-trip.
 import type Database from 'better-sqlite3';
 
 export const version = 3;