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()
+      );
+    `,
+  },
 ];

backend/src/db/schema.sql

@@ -562,6 +562,14 @@ CREATE INDEX IF NOT EXISTS idx_vibe_events_session_occurred
 CREATE INDEX IF NOT EXISTS idx_vibe_events_user_occurred
     ON vibe_events (user_id, occurred_at DESC);
 
+-- Exactly-once projection marker for material Vibe feedback. The immutable
+-- event remains authoritative; this row proves its effect was applied to the
+-- listener inputs without double-counting an idempotent client retry.
+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()
+);
+
 CREATE TABLE IF NOT EXISTS vibe_plan_versions (
     id                 UUID PRIMARY KEY DEFAULT gen_random_uuid(),
     session_id         UUID NOT NULL REFERENCES vibe_sessions(id) ON DELETE CASCADE,