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.c

@@ -77,6 +77,7 @@
  *     #pulse.default.position = [ FL FR ]
  *     #pulse.idle.timeout     = 0
  *     #pulse.max-streams      = 64
+ *     #pulse.max-sample-cache   = 67108864
  * }
  *
  * pulse.properties.rules = [
@@ -254,6 +255,13 @@
  *
  * The maximum number of streams a single client can create. Default is 64.
  *
+ *\code{.unparsed}
+ *     pulse.max-sample-cache = 67108864
+ *\endcode
+ *
+ * The maximum total size in bytes of all sample cache entries. Default is
+ * 67108864 (64MB).
+ *
  * ## Command execution
  *
  * As part of the server startup sequence, a set of commands can be executed.