test: GraphLive live mode + Show snmp_sensor format + Alert/Activity events
Bumps coverage by exercising several previously-uncovered branches:
- GraphLive.Show: new test file driving `range=live` for every
sensor_type (latency, processors, memory, traffic, temperature,
storage, voltage, count, pppoe_sessions, connections, wireless,
unknown), plus `get_title_suffix/1` across sensor_id / interface_id
/ storage_id, and `change_range` flipping into live mode.
- DeviceLive.Show: format_check_value/2 now covers the snmp_sensor
branch across temperature, voltage, current, power, frequency,
humidity, fanspeed (default + custom unit), and an unknown
sensor_type with/without a configured unit.
- ActivityFeedLive: clear_search and load_more events; PubSub
handle_info clauses for new_alert / alert_resolved / device_added
/ config_changed / sync_completed / device_event / :tick / catch-all.
- AlertLive.Index: acknowledge / resolve event handlers across
success, not-found, and cross-org unauthorized branches.
Coverage: 86.42% → 86.52%.