feat(persistence): PostgreSQL assistant store, chat persist fixes, gateway scripts

- Add SQLAlchemy Core repos, pg adapter/compat, assistant_store factory, Alembic bootstrap and migration/cutover scripts.

- Harden chat_message writes (NUL scrub for PG), turn_uuid on attempt failure, WS turn_runner fallbacks and gateway executed_turn_uuid init.

- start_gateway: log paths, PS7 stderr handling via cmd, background stdout/stderr redirect; runtime assistant_runtime_log_dir export.

- Ops: clear_all_chat_sessions with PG-only --postgresql and env-gated wipe; clear_postgres_chat_sessions.ps1.

- Tests: SA repos, pg compat, persist fallback, smoke env isolation; CI and docs touch-ups.

Co-authored-by: Cursor <cursoragent@cursor.com>

svc/persistence/db/__init__.py

@@ -0,0 +1,43 @@
+"""Persistence DB helpers."""
+
+from svc.persistence.db.engine import (
+    clear_assistant_engine_cache,
+    engine_for_sqlite_file,
+    get_assistant_engine,
+)
+from svc.persistence.db.tables import (
+    app_setting,
+    app_user,
+    auth_session,
+    bind_code,
+    channel_identity,
+    channel_identity_v2,
+    channel_session_v2,
+    chat_message,
+    chat_session,
+    metadata,
+    tenant,
+    tool_log,
+    trace_event,
+    ui_session_owner,
+)
+
+__all__ = [
+    "app_user",
+    "app_setting",
+    "auth_session",
+    "bind_code",
+    "channel_identity",
+    "channel_identity_v2",
+    "channel_session_v2",
+    "chat_message",
+    "chat_session",
+    "clear_assistant_engine_cache",
+    "engine_for_sqlite_file",
+    "get_assistant_engine",
+    "metadata",
+    "tenant",
+    "tool_log",
+    "trace_event",
+    "ui_session_owner",
+]