Two critical production bugs fixed:
1. **Alert type enum → string conversion**
- Changed Alert.alert_type from Ecto.Enum to :string for flexibility
- Updated all queries to use "device_down"/"device_up" strings instead of atoms
- Fixed pattern matching in alerts.ex and device_monitor_worker.ex
- Updated 44+ test files to use string literals
2. **SQL array indexing syntax error in activity feed**
- Fixed PostgreSQL syntax: `array_agg(...)[1]` → `(array_agg(...))[1]`
- Prevents "syntax error at or near [" in activity feed queries
3. **Added comprehensive timer cleanup tests**
- Tests for mobile_qr_live.ex timer cleanup on terminate
- Tests for agent_live index timer cleanup
- Verifies memory leak fixes from previous commits
Files changed:
- lib/towerops/alerts.ex
- lib/towerops/alerts/alert.ex
- lib/towerops/activity_feed.ex
- lib/towerops/workers/device_monitor_worker.ex
- test/**/*_test.exs (44+ files with alert_type references)
- test/towerops_web/live/mobile_qr_live_test.exs
- test/towerops_web/live/agent_live_test.exs
- test/towerops/workers/device_poller_worker_test.exs
All tests passing except 1 unrelated brute force protection test.
