UNIQUE constraint violations from rack/device/model creation and editing
now show readable messages like 'A rack named X already exists, onii-chan'
instead of raw SQLite error strings or silent redirects.
Added is_power_strip flag to device_models (schema migration, model,
form checkbox, DB CRUD). Power strip devices in rack view now render
a grid of plug-icon outlets instead of the regular port list, with
custom cable colors showing which outlets are in use.
ModelDelete now checks the DB error (e.g. FK constraint from devices still
using the model) and renders the model list with an alert instead of
silently redirecting with no feedback.
Joca