fix: tree_shares.account_id must come from tree owner, not the actor

- trees.py: change account_id=current_user.account_id →
  account_id=tree.account_id so super-admin cross-account shares land in
  the tree's tenant where RLS will see them.

- migration a05e1a1bea7c: fix backfill to join tree_shares → trees instead
  of tree_shares → users(created_by). Same logic: historical shares belong
  to the tree's tenant.

- test_tree_sharing.py: add test_share_account_id_matches_tree_not_actor
  to assert share.account_id == tree.account_id after POST /share; also
  add missing account_id to all direct TreeShare(...) constructors in
  existing tests.

- test_phase1_migrations.py: remove team_id= from TargetList constructor
  (column dropped in Phase 3).

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

backend/tests/test_phase1_migrations.py

@@ -464,7 +464,6 @@ async def test_target_list_account_id_from_team_admin(test_db: AsyncSession):
     await test_db.flush()
 
     target_list = TargetList(
-        team_id=team.id,
         account_id=account.id,
         created_by=user.id,
         name="Server Targets",

backend/tests/test_tree_sharing.py

@@ -117,6 +117,7 @@ class TestTreeSharing:
         for i in range(3):
             share = TreeShare(
                 tree_id=sample_tree.id,
+                account_id=sample_tree.account_id,
                 share_token=f"token_{i}_" + "x" * 56,
                 created_by=sample_tree.author_id,
                 allow_forking=i % 2 == 0
@@ -162,6 +163,7 @@ class TestTreeSharing:
         # Create a share
         share = TreeShare(
             tree_id=sample_tree.id,
+            account_id=sample_tree.account_id,
             share_token="public_test_token" + "x" * 47,
             created_by=UUID(test_user["user_data"]["id"]),
             allow_forking=True
@@ -192,6 +194,7 @@ class TestTreeSharing:
         # Create expired share
         share = TreeShare(
             tree_id=sample_tree.id,
+            account_id=sample_tree.account_id,
             share_token="expired_token" + "x" * 50,
             created_by=UUID(test_user["user_data"]["id"]),
             allow_forking=True,
@@ -209,6 +212,7 @@ class TestTreeSharing:
         from uuid import UUID
         share = TreeShare(
             tree_id=sample_tree.id,
+            account_id=sample_tree.account_id,
             share_token="inactive_tree_token" + "x" * 44,
             created_by=UUID(test_user["user_data"]["id"]),
             allow_forking=True
@@ -248,6 +252,37 @@ class TestTreeSharing:
             tokens.add(token)
         assert len(tokens) == 5
 
+    async def test_share_account_id_matches_tree_not_actor(
+        self, client: AsyncClient, sample_tree, auth_headers, test_db
+    ):
+        """Share account_id must equal tree.account_id, not the actor's account_id.
+
+        A super admin in a different account can share any tree. The resulting
+        TreeShare row must live in the tree-owner's account so that the tree
+        owner's RLS context covers it. If account_id were derived from the
+        actor instead, the share would vanish from the tree owner's view once
+        RLS is enabled.
+        """
+        from uuid import UUID
+        from sqlalchemy import select
+
+        response = await client.post(
+            f"/api/v1/trees/{sample_tree.id}/share",
+            json={"allow_forking": True},
+            headers=auth_headers,
+        )
+        assert response.status_code == 201
+        share_token = response.json()["share_token"]
+
+        result = await test_db.execute(
+            select(TreeShare).where(TreeShare.share_token == share_token)
+        )
+        share = result.scalar_one()
+        assert share.account_id == sample_tree.account_id, (
+            "TreeShare.account_id must equal tree.account_id, not the actor's account. "
+            "Shares must live in the tree owner's tenant for RLS to cover them."
+        )
+
 
 @pytest.mark.asyncio
 async def test_migration_defaults_visibility_to_team(test_db):