Ciaran/storage management

[ciaranbor]

Motivation

• Adds storage management to exo so nodes can enforce disk usage limits for downloaded models
• Prevents nodes from filling up their disks with model downloads, with configurable per-node limits and eviction policies

Changes

• New types: StorageConfig (max storage + eviction policy), DownloadRejected download status, StorageConfigUpdated event, SetStorageConfig command
• State/apply: StorageConfig added to cluster state; apply handles StorageConfigUpdated events
• Storage utilities (shared/storage.py): pure functions for calculating used storage, checking quotas, computing LRU eviction candidates
• Download coordinator: checks storage quota before starting downloads; rejects if over limit (manual policy) or auto-evicts oldest unused models (auto-evict policy); tracks model usage timestamps via a TOML file; persists config to
  ~/.cache/exo/config.toml
• Master: handles SetStorageConfig command, emits StorageConfigUpdated event
• API: new PUT /storage/config endpoint for setting per-node storage config
• CLI: --max-storage and --eviction-policy arguments
• Dashboard: storage bar per node in downloads page header, gear icon opens a settings modal to configure max storage and eviction policy, DownloadRejected cells shown with orange warning icon and retry button

Why It Works

• Storage checks happen at download-request time in the coordinator, so downloads are rejected/evicted before disk space runs out
• LRU eviction uses tracked usage timestamps (updated on inference) so actively-used models are preserved
• Config persists to disk so it survives restarts; state is also broadcast via event sourcing so the dashboard reflects current settings

Test Plan

Manual Testing

• Configure storage limits via dashboard modal; verify downloads are rejected when over limit and auto-evicted when using auto-evict policy

Automated Testing

• test_storage.py: 336 lines covering calculate_used_storage, check_storage_quota, compute_evictions_needed, get_lru_eviction_candidates with edge cases
• test_apply_storage.py: tests StorageConfigUpdated event application to state
• test_auto_eviction.py: 409 lines testing the coordinator's eviction logic including LRU ordering, multi-model eviction, and insufficient-space scenarios