feat: tenant isolation Phase 4 — RLS on 31 remaining tables + script_builder fix

Enable RLS on all remaining tenant-scoped tables (31 tables):

Standard policy (tenant sees own rows):
  users, account_invites, account_limit_overrides, account_feature_overrides,
  subscriptions, ai_chat_sessions, ai_conversations, ai_session_steps,
  ai_session_embeddings, ai_suggestions, ai_usage, assistant_chats,
  attachments, copilot_conversations, feedback, file_uploads, fork_points,
  kb_imports, notifications, notification_configs, notification_logs,
  psa_activity_logs, psa_member_mappings, script_builder_sessions,
  script_categories, session_ratings, tree_embeddings, user_folders,
  user_pinned_trees

Platform-visibility policy (own rows OR PLATFORM_ACCOUNT_ID):
  platform_steps, template_trees

Intentionally skipped:
  accounts (IS the root table, no account_id column)
  plan_feature_defaults (platform config, no account_id column)

Also fixes script_builder_service.create_session() which was missing
account_id= on ScriptBuilderSession construction, causing 500s on all
script builder endpoints (pre-existing CI failure).

Adds Phase 4 RLS isolation tests covering: users, script_builder_sessions,
ai_session_steps, notifications, platform_steps, template_trees.

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

backend/app/api/endpoints/script_builder.py

@@ -85,6 +85,7 @@ async def create_session(
     session = await script_builder_service.create_session(
         db=db,
         user_id=current_user.id,
+        account_id=current_user.account_id,
         team_id=current_user.team_id,
         language=data.language,
     )