feat(proxy-engine): add device leg, leg transfer, and leg replacement call controls

ts/frontend.ts

@@ -128,14 +128,19 @@ async function handleRequest(
     }
   }
 
-  // API: add leg to call (device — not yet implemented, needs device-to-call routing).
+  // API: add a SIP device to a call (mid-call INVITE to desk phone).
   if (url.pathname.startsWith('/api/call/') && url.pathname.endsWith('/addleg') && method === 'POST') {
     try {
       const callId = url.pathname.split('/')[3];
       const body = await readJsonBody(req);
       if (!body?.deviceId) return sendJson(res, { ok: false, error: 'missing deviceId' }, 400);
-      // TODO: implement device leg addition (needs SIP INVITE to device).
-      return sendJson(res, { ok: false, error: 'not yet implemented' }, 501);
+      const { addDeviceLeg } = await import('./proxybridge.ts');
+      const legId = await addDeviceLeg(callId, body.deviceId);
+      if (legId) {
+        return sendJson(res, { ok: true, legId });
+      } else {
+        return sendJson(res, { ok: false, error: 'device not registered or call not found' }, 404);
+      }
     } catch (e: any) {
       return sendJson(res, { ok: false, error: e.message }, 400);
     }