feat(backend): add automatic DB migrations

Add a lightweight migration runner with schema_migrations tracking, run pending migrations during backend startup before the scheduler, and keep a manual backend-migrate entrypoint.

The change also moves the existing lockout and task-thread-ID schema steps into shared migration modules, updates docs, and archives the OpenSpec change.

tests/test_run_migrations_script.py

@@ -0,0 +1,36 @@
+from __future__ import annotations
+
+import importlib.util
+from pathlib import Path
+from types import SimpleNamespace
+from unittest.mock import patch
+
+SCRIPT_PATH = (
+    Path(__file__).resolve().parents[1] / "apps" / "backend" / "scripts" / "run_migrations.py"
+)
+SCRIPT_SPEC = importlib.util.spec_from_file_location("backend.scripts.run_migrations", SCRIPT_PATH)
+assert SCRIPT_SPEC is not None and SCRIPT_SPEC.loader is not None
+run_migrations = importlib.util.module_from_spec(SCRIPT_SPEC)
+SCRIPT_SPEC.loader.exec_module(run_migrations)
+
+
+def test_run_migrations_initializes_database_before_running_pending_migrations() -> None:
+    calls: list[str] = []
+
+    def fake_init_db() -> None:
+        calls.append("init_db")
+
+    def fake_run_pending_migrations():
+        calls.append("run_pending_migrations")
+        return SimpleNamespace(applied=("001",), skipped=())
+
+    with (
+        patch.object(run_migrations, "init_db", side_effect=fake_init_db),
+        patch.object(
+            run_migrations, "run_pending_migrations", side_effect=fake_run_pending_migrations
+        ),
+    ):
+        exit_code = run_migrations.main()
+
+    assert exit_code == 0
+    assert calls == ["init_db", "run_pending_migrations"]