Phase 3: OpenSubsonic core (response envelope, error codes 42-44, getOpenSubsonicExtensions) and apiKeyAuthentication extension with tokenInfo, web API key management

internal/subsonic/apikey.go

@@ -0,0 +1,15 @@
+package subsonic
+
+func init() {
+	registerExtension("apiKeyAuthentication", 1)
+	register("tokenInfo", handleTokenInfo)
+}
+
+// handleTokenInfo confirms which user an API key (or any other
+// credential) resolves to — primarily useful for a client that just
+// received a key and wants to confirm it works before storing it.
+func handleTokenInfo(rc *Context) *apiError {
+	rc.Response = newResponse()
+	rc.Response.TokenInfo = &TokenInfo{Username: rc.User.Username}
+	return nil
+}