fix: resolve CI failures — SQLAlchemy mapper conflict and lint errors

- Add back_populates to SessionBranch.session ↔ AISession.branches
  to fix SQLAlchemy InvalidRequestError that broke all backend tests
- Remove unused 'tree' variable in command-palette.spec.ts
- Suppress setState-in-effect lint rule in TaskLane (intentional sync)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/e2e/command-palette.spec.ts

@@ -29,7 +29,7 @@ test.describe('command palette smoke tests', () => {
 
   test('searches and shows AI Assistant option', async ({ page }) => {
     const api = await createAuthenticatedApiContext()
-    const tree = await createTroubleshootingTree(api, {
+    await createTroubleshootingTree(api, {
       name: uniqueName('PW Palette Search Flow'),
     })