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/alembic/versions/04f013768235_enable_rls_phase3.py

@@ -0,0 +1,59 @@
+"""Enable RLS on Phase 3 tables.
+
+Tables covered:
+  - step_ratings      (account_id NOT NULL since migration 7167e9374b0c)
+  - step_usage_log    (account_id NOT NULL since migration 7167e9374b0c)
+  - target_lists      (account_id NOT NULL since migration 2c6aabd89bc6)
+  - session_shares    (account_id NOT NULL since session_share model)
+  - audit_logs        (account_id NOT NULL since migration 2a9056eddd90)
+  - tree_shares       (account_id NOT NULL since migration a05e1a1bea7c)
+
+All use a standard intra-tenant isolation policy.
+Token-based access to session_shares and tree_shares goes through
+endpoints that use get_admin_db (BYPASSRLS), so a strict tenant
+policy here is correct.
+
+Revision ID: 04f013768235
+Revises: a05e1a1bea7c
+Create Date: 2026-04-11 00:00:00.000000
+"""
+from typing import Sequence, Union
+from alembic import op
+
+revision: str = '04f013768235'
+down_revision: Union[str, None] = 'a05e1a1bea7c'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+_CURRENT_ACCOUNT = (
+    "COALESCE(NULLIF(current_setting('app.current_account_id', TRUE), ''), "
+    "'00000000-0000-0000-0000-000000000000')::uuid"
+)
+
+_STANDARD_USING = f"account_id = {_CURRENT_ACCOUNT}"
+
+_PHASE3_TABLES = [
+    "step_ratings",
+    "step_usage_log",
+    "target_lists",
+    "session_shares",
+    "audit_logs",
+    "tree_shares",
+]
+
+
+def upgrade() -> None:
+    for table in _PHASE3_TABLES:
+        op.execute(f"ALTER TABLE {table} ENABLE ROW LEVEL SECURITY")
+        op.execute(f"ALTER TABLE {table} FORCE ROW LEVEL SECURITY")
+        op.execute(f"""
+            CREATE POLICY tenant_isolation ON {table}
+                USING ({_STANDARD_USING})
+        """)
+
+
+def downgrade() -> None:
+    for table in _PHASE3_TABLES:
+        op.execute(f"DROP POLICY IF EXISTS tenant_isolation ON {table}")
+        op.execute(f"ALTER TABLE {table} DISABLE ROW LEVEL SECURITY")
+        op.execute(f"ALTER TABLE {table} NO FORCE ROW LEVEL SECURITY")