feat: Phase 1 Group 2 — add account_id to AI branching tables

Tables: session_branches, session_handoffs, fork_points,
        ai_session_steps, ai_suggestions
Backfill: session_id → ai_sessions.account_id (all except
ai_suggestions which uses user_id → users.account_id)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_phase1_migrations.py

@@ -160,3 +160,64 @@ async def test_session_resolution_output_account_id(test_db: AsyncSession):
     )
     row = result.scalar_one()
     assert row.account_id == account.id
+
+
+# ── Group 2: AI & branching ───────────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_session_branch_account_id_matches_ai_session(test_db: AsyncSession):
+    """session_branches.account_id must match parent ai_session.account_id."""
+    from app.models.session_branch import SessionBranch
+
+    account, user = await _make_account_and_user(test_db, "sb1")
+    ai_session = AISession(
+        user_id=user.id,
+        account_id=account.id,
+        problem_summary="test",
+        problem_domain="networking",
+        status="active",
+    )
+    test_db.add(ai_session)
+    await test_db.flush()
+
+    branch = SessionBranch(
+        session_id=ai_session.id,
+        account_id=account.id,
+        label="Branch A",
+        branch_order=1,
+        conversation_messages=[],
+    )
+    test_db.add(branch)
+    await test_db.commit()
+
+    result = await test_db.execute(
+        select(SessionBranch).where(SessionBranch.id == branch.id)
+    )
+    row = result.scalar_one()
+    assert row.account_id == account.id
+
+
+@pytest.mark.asyncio
+async def test_ai_suggestion_account_id_matches_user(test_db: AsyncSession):
+    """ai_suggestions.account_id must match the creating user's account_id."""
+    from app.models.ai_suggestion import AISuggestion
+
+    account, user = await _make_account_and_user(test_db, "ais1")
+    tree = await _make_tree(test_db, account, user)
+
+    suggestion = AISuggestion(
+        tree_id=tree.id,
+        user_id=user.id,
+        account_id=account.id,
+        action_type="add_node",
+        changes_json={},
+        status="pending",
+    )
+    test_db.add(suggestion)
+    await test_db.commit()
+
+    result = await test_db.execute(
+        select(AISuggestion).where(AISuggestion.id == suggestion.id)
+    )
+    row = result.scalar_one()
+    assert row.account_id == account.id