feat(vibe): add durable versioned session API

backend/src/db/migrations.ts

@@ -708,4 +708,16 @@ export const MIGRATIONS: Migration[] = [
       );
     `,
   },
+  {
+    // A material Vibe event is projected into the legacy listener inputs in
+    // the same transaction as its ledger write. This marker makes that bridge
+    // auditable and exactly-once even when a client retries an event id.
+    id: '20260801_vibe_event_projections',
+    sql: `
+      CREATE TABLE IF NOT EXISTS vibe_event_projections (
+        event_id UUID PRIMARY KEY REFERENCES vibe_events(id) ON DELETE CASCADE,
+        projected_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+      );
+    `,
+  },
 ];