feat(ntfy): add the HTTP client, stream manager, and account discovery

Speak the ntfy JSON API directly: subscribe, publish, group topics that share a server and credentials, reconnect with backoff and a since cursor, and list account subscriptions, reservations, and ACL grants.

internal/ntfy/scan.go

@@ -0,0 +1,13 @@
+package ntfy
+
+import (
+	"bufio"
+	"io"
+)
+
+func newScanner(r io.Reader) *bufio.Scanner {
+	sc := bufio.NewScanner(r)
+	buf := make([]byte, 64*1024)
+	sc.Buffer(buf, 10*1024*1024)
+	return sc
+}