security: add total sample cache size limit in PulseAudio protocol

There was no limit on the total size of the sample cache. A client
could upload many samples to exhaust server memory. Add a configurable
pulse.max-sample-cache property (default 64MB) to cap the total size
of all cached samples.

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

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

@@ -38,6 +38,7 @@
 #define MAX_NAME		1024u
 
 #define SCACHE_ENTRY_SIZE_MAX	(1024*1024*16)
+#define MAX_SAMPLE_CACHE	(1024u*1024*64)	/* 64MB */
 
 #define MAX_CLIENTS		64u
 #define MAX_STREAMS		64u