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/library/mediainfo/ffprobe.go

@@ -78,6 +78,12 @@ func ExtractMediaInfo(ctx context.Context, ffprobePath, filePath string) (*Media
 		return nil, fmt.Errorf("ffprobe JSON parse failed: %w", err)
 	}
 
+	return parseFFprobeOutput(data)
+}
+
+// parseFFprobeOutput converts parsed ffprobe JSON into MediaInfo.
+// Separated from ExtractMediaInfo so it can be tested without running ffprobe.
+func parseFFprobeOutput(data ffprobeOutput) (*MediaInfo, error) {
 	if len(data.Streams) == 0 {
 		return nil, fmt.Errorf("ffprobe returned no streams")
 	}