feat: tenant isolation Phase 3 — audit_logs, tree_shares, remaining RLS

P3-A: Add account_id to audit_logs model + migration (backfill via user_id →
  users.account_id). log_audit() gains optional account_id param with fallback
  SELECT to avoid churn across 40 call sites.

P3-B: Add account_id to tree_shares model + migration (backfill via created_by
  → users.account_id). TreeShare constructor updated in trees.py.

P3-C: Enable RLS on 6 remaining tables: step_ratings, step_usage_log,
  target_lists, session_shares, audit_logs, tree_shares.

P3-D: Drop team_id from target_lists — endpoint, schema, and model now use
  account_id as the sole isolation key.

P3-E: Append Phase 3 RLS isolation tests for all 6 tables.

test_target_lists.py: fix cross-account test to use Account model (not Team)
and set account_id on new User.

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

backend/app/models/target_list.py

@@ -8,7 +8,6 @@ from app.core.database import Base
 
 if TYPE_CHECKING:
     from app.models.user import User
-    from app.models.team import Team
     from app.models.account import Account
 
 
@@ -18,10 +17,6 @@ class TargetList(Base):
     id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
     )
-    team_id: Mapped[uuid.UUID] = mapped_column(
-        UUID(as_uuid=True), ForeignKey("teams.id", ondelete="CASCADE"),
-        nullable=False, index=True
-    )
     account_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("accounts.id", ondelete="CASCADE"),