feat: honor X-Jambonz-SRTP header for forwarded sip URI calls (#230)

The feature-server sets X-Jambonz-SRTP (from the dial verb's
srtpEncryption option) to request encrypted media on a per-call basis.
Previously SRTP on a forwarded sip URI could only be enabled globally
via JAMBONES_SIPS_FORWARD_SRTP + a sips: scheme; now an application can
opt in per call. The env var remains as a global fallback. The internal
header is stripped before the INVITE is sent to the target.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/call-session.js

@@ -237,7 +237,20 @@ class CallSession extends Emitter {
         this.logger.info(`sending call to registered user ${destUri}`);
       }
       else if (this.req.locals.target === 'forward') {
-        if (process.env.JAMBONES_SIPS_FORWARD_SRTP && this.req.uri.startsWith('sips:')) {
+        /* the feature-server sets X-Jambonz-SRTP (from the dial verb's srtpEncryption
+           option) to request encrypted media on a per-call basis */
+        const srtpMode = this.req.get('X-Jambonz-SRTP');
+        if (srtpMode) {
+          /* SDES (RTP/SAVP): pass teams=true to select the SDES srtp profile, matching
+             the proven carrier tls/srtp path above. This offers a=crypto to the target
+             (what SIP endpoints such as LiveKit expect), not DTLS-SRTP. */
+          this.logger.info({uri: this.req.uri, srtpMode},
+            'using SRTP (SDES) for forwarded call per X-Jambonz-SRTP');
+          this.rtpEngineOpts = makeRtpEngineOpts(this.req, false, true, false, true);
+          encryptedMedia = true;
+        }
+        else if (process.env.JAMBONES_SIPS_FORWARD_SRTP && this.req.uri.startsWith('sips:')) {
+          /* legacy global opt-in for sips: forwards (uses the DTLS srtp profile) */
           this.logger.info({uri: this.req.uri}, 'using SRTP/TLS for forwarded sips: call');
           this.rtpEngineOpts = makeRtpEngineOpts(this.req, false, true, false, false);
           encryptedMedia = true;
@@ -531,6 +544,7 @@ class CallSession extends Emitter {
               '-X-Preferred-From-Host',
               '-X-Jambonz-FS-UUID',
               '-X-Voip-Carrier-Sid',
+              '-X-Jambonz-SRTP',
               '-X-SIP-Proxy'
             ],
             proxyResponseHeaders: [