Add trusted device management API

[alceops]

Closes #125

Summary

• records device identity and metadata on successful login
• adds authenticated endpoints to list, rename/trust, and revoke devices
• adds PostgreSQL compatibility DDL plus Redis outage fallback so auth/session/cache paths keep working in local/free-tier environments
• invalidates dashboard cache after expense creation to avoid stale summaries when Redis fallback is active

Tests

• PYTHONPATH=. ../../.venv/bin/pytest tests