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/root.go

@@ -143,8 +143,9 @@ Source:         https://github.com/torrentclaw/unarr`,
 		completionCmd,
 		// Library
 		scanCmd,
+		// Alias: upgrade → self-update
+		newUpgradeCmd(),
 		// Stubs for future commands
-		newStubCmd("upgrade", "Find a better version of a torrent"),
 		newStubCmd("moreseed", "Find same quality with more seeders"),
 		newStubCmd("compare", "Compare two torrents side by side"),
 		newStubCmd("add", "Search and add torrents to your client"),