security: add per-client stream limit in PulseAudio protocol

There was no limit on the number of streams a single client could
create. Each stream allocates a 4MB ring buffer, allowing a malicious
client to exhaust server memory. Add a configurable pulse.max-streams
property (default 64) to limit streams per client.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/module-protocol-pulse/internal.h

@@ -36,6 +36,7 @@ struct defs {
 	struct channel_map channel_map;
 	uint32_t quantum_limit;
 	uint32_t idle_timeout;
+	uint32_t max_streams;
 };
 
 struct stats {