fix(billing): read Goose accumulated_cost from PostgreSQL fallback

Goosed GET /sessions/:id returns accumulated_cost null while memind_sessions
PG has the real upstream cost. Fall back to PG before token estimate billing.

Co-authored-by: Cursor <cursoragent@cursor.com>

goose-session-cost.test.mjs

@@ -0,0 +1,30 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  isGooseSessionPgConfigured,
+  resolveGooseSessionPgUrl,
+} from './goose-session-cost.mjs';
+
+test('resolveGooseSessionPgUrl prefers GOOSE_SESSION_DB_URL', () => {
+  assert.equal(
+    resolveGooseSessionPgUrl({ GOOSE_SESSION_DB_URL: 'postgresql://u:p@host:5432/db' }),
+    'postgresql://u:p@host:5432/db',
+  );
+});
+
+test('resolveGooseSessionPgUrl builds local default DSN', () => {
+  assert.equal(
+    resolveGooseSessionPgUrl({
+      GOOSE_SESSION_PG_HOST: '127.0.0.1',
+      GOOSE_SESSION_PG_PORT: '5432',
+      GOOSE_SESSION_PG_DATABASE: 'memind_sessions',
+      GOOSE_SESSION_PG_USER: 'john',
+    }),
+    'postgresql://john@127.0.0.1:5432/memind_sessions',
+  );
+});
+
+test('isGooseSessionPgConfigured can be disabled explicitly', () => {
+  assert.equal(isGooseSessionPgConfigured({ GOOSE_SESSION_PG_DISABLE: '1' }), false);
+  assert.equal(isGooseSessionPgConfigured({ GOOSE_SESSION_DB_URL: 'postgresql://x' }), true);
+});