## Summary
- **DB pool exhaustion**: `device_has_effective_agent?` was doing 2 DB queries per call (3-table join + global settings lookup), called 2-3x per worker cycle across hundreds of devices (~6000 unnecessary queries/minute). Added an ETS-backed `AgentCache` with 120s TTL and removed redundant calls within the same worker cycle.
- **Settings page crash**: `KeyError: key :user_agent not found` on BrowserSession — field doesn't exist, replaced with `device_type`.
- **Login history IP truncation**: Changed from side-by-side grid to stacked table layout so IP addresses aren't clipped.
## Test plan
- [x] 8 new tests for AgentCache (cache hit, miss, invalidation, global default)
- [x] Full test suite passes (8651 tests, 0 failures, verified across multiple runs)
- [x] `mix format` and pre-commit hooks pass
Reviewed-on: graham/towerops-web#88
- Downgrade check executor skip log from info to debug to stop flooding
production logs every 60s for every agent-assigned device check
- Fix KeyError on session.user_agent in settings page - BrowserSession
schema has device_type field, not user_agent
Reviewed-on: graham/towerops-web#87
Add an Appearance section to the Personal tab in user settings with
three clearly labelled options (Auto/Light/Dark). A ThemeSelector hook
reads localStorage to highlight the active choice and updates it when
selection changes. Remove the dropdown toggle from both the main nav
and admin nav.
wire time_format into scope, add settings UI selector, change defaults
to 24h, and replace all user-facing Calendar.strftime calls with
centralized TimeHelpers using the user's timezone and time format.
