Maintenance: reload settings when the file changes so audits reflect current cache paths

[Brandon-Haney]

Fixes #176.

Problem

The Maintenance/audit layer (MaintenanceService) is a process-lifetime singleton that memoized settings and the derived cache/array directory paths on first use, with no invalidation. If path mappings were edited after the web process started (e.g. a mapping gained its cache_path), the audit kept scanning the previous path.

When that cached path resolved to an empty or old directory, the cache scan returned 0 files, so the audit marked every tracked exclude and timestamp entry as "stale — no longer on cache." The dashboard then showed a large warning count (e.g. 140 warnings = 70 stale exclude + 70 stale timestamp) and Maintenance showed Protected: N of 0 on cache. Cleaning removed the tracking entries (the physical files were untouched), and the next run re-created them, so the warnings returned every run. A container restart cleared the stale snapshot and the warnings stayed gone — confirming the cause.

The actual caching run was always correct: it resolves each file via mapping.cache_path and found all cached files. Only the long-lived audit singleton was out of date.

Fix

_load_settings() now tracks the settings file mtime and reloads when it changes, clearing the derived cache_dirs/array_dirs so they are recomputed from the current configuration. _get_paths() consults _load_settings() before its cache short-circuit so the reload is honored. When settings are unchanged, behavior is identical to before (no extra file reads beyond a cheap stat).

Testing

1. Start the Web UI and run a cache operation so files are tracked and the dashboard shows them as cached.
2. Edit a path mapping in Settings (or complete setup so a mapping gains its cache path) without restarting.
3. Open Maintenance / Dashboard and confirm the audit reflects the updated mapping — cached files are counted and no false "stale" warnings appear.
4. Confirm that with mappings unchanged, repeated runs keep a stable tracking count and an empty warning list.

Automated coverage added in tests/test_maintenance_service.py (TestSettingsReloadOnChange): paths refresh after a settings change, stay cached when unchanged, and the cache scan recovers once a mapping is corrected.