Discovery Fixes:
- Always update cache even when scan finds no servers (prevents stale data)
- Remove automatic re-add of deleted nodes to discovery (was causing confusion)
- Optimize Docker subnet scanning from 762 IPs to 254 IPs (3x faster)
- Add getHostSubnetFromGateway() to detect host network from container
Frontend Type Fixes:
- Fix ThresholdsTable editScope type errors
- Fix SnapshotAlertConfig index signature
- Remove unused variable in Settings.tsx
These changes make discovery faster, more reliable, and fix the issue where
deleted nodes would persist in the discovery cache or immediately reappear.
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