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/server.c

@@ -45,7 +45,6 @@
 #endif
 
 #define LISTEN_BACKLOG 32
-#define MAX_CLIENTS 64
 
 PW_LOG_TOPIC_EXTERN(pulse_conn);