chore: improve memU maintenance path

[lphuc2250gma]

Summary:

• Add or tighten focused edge-case tests or type assertions in src/memu/app/crud.py, tests/test_sqlite.py related to Python typing, tests, CLI ergonomics, observability; avoid docs-only changes and broad refactors.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.