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/engine/method_test.go

@@ -0,0 +1,50 @@
+package engine
+
+import "testing"
+
+func TestDownloadMethodConstants(t *testing.T) {
+	if MethodTorrent != "torrent" {
+		t.Errorf("MethodTorrent = %q, want torrent", MethodTorrent)
+	}
+	if MethodDebrid != "debrid" {
+		t.Errorf("MethodDebrid = %q, want debrid", MethodDebrid)
+	}
+	if MethodUsenet != "usenet" {
+		t.Errorf("MethodUsenet = %q, want usenet", MethodUsenet)
+	}
+}
+
+func TestProgressStruct(t *testing.T) {
+	p := Progress{
+		DownloadedBytes: 1024,
+		TotalBytes:      2048,
+		SpeedBps:        512,
+		ETA:             10,
+		Peers:           5,
+		Seeds:           3,
+		FileName:        "movie.mkv",
+	}
+
+	if p.DownloadedBytes != 1024 {
+		t.Errorf("DownloadedBytes = %d, want 1024", p.DownloadedBytes)
+	}
+	if p.FileName != "movie.mkv" {
+		t.Errorf("FileName = %q, want movie.mkv", p.FileName)
+	}
+}
+
+func TestResultStruct(t *testing.T) {
+	r := Result{
+		FilePath: "/downloads/movie.mkv",
+		FileName: "movie.mkv",
+		Method:   MethodTorrent,
+		Size:     1073741824,
+	}
+
+	if r.Method != MethodTorrent {
+		t.Errorf("Method = %q, want torrent", r.Method)
+	}
+	if r.Size != 1073741824 {
+		t.Errorf("Size = %d, want 1073741824", r.Size)
+	}
+}