feat(cli): upgrade command, rich status, and version cache

- Replace `upgrade` stub with real command (alias for `self-update`)
- Also register `update` as alias: `unarr update` works too
- Rewrite `status` to show full config, disk usage, daemon state, and
  update availability with colored sections
- Add version check cache (1h TTL) so `status` is instant on repeat runs
- Guard against division by zero on empty filesystems
- Guard against negative durations from clock skew
- Guard against stale PID via heartbeat recency check (2 min)
- Add comprehensive test coverage across agent, engine, upgrade, usenet,
  arr, library, mediaserver, and UI packages
- Improve Makefile coverage target to exclude cmd/ glue code
- Fix stream handler resource cleanup and ffprobe error handling

internal/cmd/upgrade.go

@@ -0,0 +1,30 @@
+package cmd
+
+import (
+	"github.com/spf13/cobra"
+)
+
+// newUpgradeCmd creates the `unarr upgrade` command as an alias for `self-update`.
+func newUpgradeCmd() *cobra.Command {
+	var force bool
+
+	cmd := &cobra.Command{
+		Use:     "upgrade",
+		Aliases: []string{"update"},
+		Short:   "Update unarr to the latest version",
+		Long: `Download and install the latest version of unarr.
+
+This is an alias for 'unarr self-update'. Checks GitHub for the latest
+release, verifies the checksum, and replaces the current binary.
+A backup is kept at <binary>.backup.`,
+		Example: `  unarr upgrade
+  unarr upgrade --force`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return runSelfUpdate(force)
+		},
+	}
+
+	cmd.Flags().BoolVarP(&force, "force", "f", false, "reinstall even if already up to date")
+
+	return cmd
+}