[Starosdev]

Summary

Adds one setting, metrics.filesystem_usage_threshold_percent (0 disables, 1 to 100), and a usage alert for Btrfs filesystems and ZFS pools on top of the per-upload notification path from #1127 and #1131. This is the optional item from #1110.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)
• CI/CD or infrastructure change

Related Issues

Closes #1132

Changes Made

• models/settings.go: FilesystemUsageThresholdPercent under metrics; MaxFilesystemUsageThresholdPercent = 100. Zero is the disabled sentinel, so no default is needed and existing installs are unchanged.
• handler/save_settings.go: rejects values outside 0..100 with a 400.
• notify/btrfs.go, notify/zfs.go: new least-severe failure types BtrfsUsageHigh and ZFSUsageHigh. BtrfsUsagePercent mirrors the dashboard formula (statfs_used / (statfs_used + statfs_available), falling back to used / device_size); ZFS uses capacity_percent. Shared usageCrossed fires at or above the threshold when newly crossed, first seen, or repeating. Both messages now carry a Usage line.
• handler/state_notify.go: settings load on every unsilenced upload (the threshold lives there) instead of after a repeat-only pre-check. Muted and archived rows still return before the load. failureTypes takes the threshold.
• Frontend: filesystem_usage_threshold_percent in app.config.ts; number input with validation in the dashboard settings dialog under the temperature settings; Save disabled while invalid via the new notificationSettingsInvalid getter.
• Docs: Btrfs and ZFS monitoring notification sections, openapi.yaml settings schema.

Testing

• I have tested this locally
• I have added/updated tests that prove my fix/feature works
• All existing tests pass

go test ./webapp/backend/pkg/web/handler/ ./webapp/backend/pkg/models/ -count=1 ok; go test ./webapp/backend/pkg/notify/ -run 'Btrfs|ZFS' ok.

• notify: 7 new Btrfs and 5 new ZFS usage cases in the failure-type tables (cross, at threshold, stays above with and without repeat, first upload, below, disabled), plus TestBtrfsUsagePercent. Full notify package still has the macOS-only TestScriptStreamsOutputAndPreservesEnvironment failure noted in feat(btrfs): notify on degraded filesystems, device errors and scrub errors #1127; unrelated.
• handler: TestSaveSettingsFilesystemUsageThresholdValidation (0, 1, 100 accepted; -1, 101 rejected). Healthy-upload tests now return settings and assert GetNotifyUrls is never called.
• Frontend: one new spec case for the threshold validation getter. Not run locally (no node_modules here); relies on the Test Frontend CI job.
• go build ./webapp/backend/..., go vet and gofmt -l clean on the touched packages.

Review: not required (lands on develop; reaches users through the promotion PR and release).

Checklist

• My code follows the project's code style
• I have performed a self-review of my code
• I have commented my code where necessary (particularly complex areas)
• I have updated the documentation if needed
• My changes generate no new warnings
• No console.log, debug statements, or commented-out code

Screenshots (if applicable)

N/A

Additional Notes

Cost: one extra LoadSettings (a setting_entries table read) per Btrfs or ZFS upload, so once per collector interval per filesystem or pool.