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/daemon/pipewire-pulse.conf.in

@@ -119,6 +119,7 @@ pulse.properties = {
     #pulse.default.tlength  = 96000/48000   # 2 seconds
     #pulse.min.quantum      = 256/48000     # 5.3ms
     #pulse.idle.timeout     = 0             # don't pause after underruns
+    #pulse.max-streams      = 64            # max streams per client
     #pulse.default.format   = F32
     #pulse.default.position = [ FL FR ]
 }