feat: add ZFS monitoring (#2209)

- track pool capacity, health, I/O, scrub status, and vdev errors
- report dataset usage and correct ZFS filesystem metrics
- add pool charts, detail views, refresh controls, and health alerts
- persist pool details and include ZFS usage in disk alerts
- support configurable detail intervals and legacy agent compatibility

---------

Co-authored-by: hank <hank@henrygd.me>

internal/hub/systems/systems_production.go

@@ -7,3 +7,6 @@ package systems
 // The hub integration tests create/replace systems and clean up the test apps quickly.
 // Background SMART fetching can outlive teardown and crash in PocketBase internals (nil DB).
 func backgroundSmartFetchEnabled() bool { return true }
+
+// Background ZFS fetching follows the same policy as SMART fetching.
+func backgroundZfsFetchEnabled() bool { return true }