feat: finish the upload, playback, and test gaps from the docs

Quality upgrades, batch covers, staging preview, and resume after reload are in the queue. Loudness and crossfade work on web and Android, and the documented checks now have suites.

Co-authored-by: Cursor <cursoragent@cursor.com>

services/api/internal/uploads/quality_test.go

@@ -0,0 +1,21 @@
+package uploads
+
+import "testing"
+
+func TestBetterQuality(t *testing.T) {
+	if !BetterQuality("flac", "flac", 900000, "mp3", "mp3", 320000) {
+		t.Fatal("flac should replace mp3")
+	}
+	if BetterQuality("mp3", "mp3", 128000, "flac", "flac", 900000) {
+		t.Fatal("mp3 is not an upgrade over flac")
+	}
+	if !BetterQuality("mp3", "mp3", 320000, "mp3", "mp3", 128000) {
+		t.Fatal("much higher bitrate is an upgrade")
+	}
+	if BetterQuality("mp3", "mp3", 180000, "mp3", "mp3", 160000) {
+		t.Fatal("small bitrate step is not an upgrade")
+	}
+	if BetterQuality("flac", "flac", 0, "flac", "flac", 0) {
+		t.Fatal("same class without bitrate is not an upgrade")
+	}
+}