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/app/api/endpoints/trees.py

@@ -1048,7 +1048,7 @@ async def create_tree_share(
     # Create share
     tree_share = TreeShare(
         tree_id=tree.id,
-        account_id=current_user.account_id,
+        account_id=tree.account_id,  # share belongs to the tree's tenant, not the actor
         share_token=share_token,
         created_by=current_user.id,
         allow_forking=share_data.allow_forking,