feat(alerts): per-device mute/snooze with timeframe

[proofofprints]

What

Silence alerts for a device that's intentionally down (e.g. a mobile miner turned off on purpose) without removing it. Priority feature of the v1.6 batch.

Backend

• New commands/mute.rs: muted_devices.json map keyed by IP (ASIC) / device_id (OverMobile), value { mutedUntil: Option<i64> }. None = until re-enabled; Some(epoch_ms) auto-expires. Persisted with an atomic write (tmp + rename).
• Commands set_device_mute(id, mutedUntil), clear_device_mute(id), get_muted_devices() — registered in lib.rs. get_muted_devices prunes expired entries (auto-clear).
• Alert engine: check_alerts + check_mobile_alerts skip muted devices — no alert, no notification, no cloud push; expired mutes excluded automatically.

Frontend

• MuteControl.tsx dropdown: 1h · 4h · 24h · Custom date/time · Until I re-enable, doubling as the Muted until … badge so it's obvious why a device is quiet.
• Wired into the ASIC (MinerDetail, keyed on IP) and OverMobile (MobileMinerDetail, keyed on deviceId) detail pages.

Verification

• cargo check ✅ · npx tsc --noEmit ✅
• Runtime: user to confirm a muted device stops firing alerts and the badge shows the expiry. (Sandbox can't see the dev binary's muted_devices.json writes.)

Deferred (nice-to-have from the spec)

• Central 'Muted devices' list / filter on the Alerts screen — each device is already clearable from its detail page; can follow up if wanted.