fix(migrations): retry temporary sqlite lock errors (#789)

[cloudyun888]

Summary

• retry migration startup when SQLite reports a temporary lock
• only retry lock-related sqlite3.OperationalError cases
• add a regression test for a transient lock on the migrations table

Testing

• pytest --noconftest tests/test_migrate.py -q
• python3 -m py_compile llm/migrations.py tests/test_migrate.py

Fixes #789