softphone: SIP.js engine, /api/me/webrtc, minimal dock

GET /api/me/webrtc returns the <ext>-web identity, WebSocket URL
(WEBRTC_WS_URL overrides wss://PUBLIC_HOSTNAME/ws) and STUN/TURN servers.
SoftphoneEngine wraps SIP.js 0.21.2: register with backoff reconnect,
call, answer, decline, hang up, remote audio. A minimal dock on /me
enables it (asks for the microphone on the click) and dials/answers;
M5-T2 grows it into the full panel.

Generator: ice_host_candidates keeps the LAN address
(include_local_address), so LAN browsers don't need hairpin NAT, and -web
endpoints set from_domain = selfphoneng.invalid because SIP.js drops
INVITEs whose From host starts with a digit (Docker/VPS hostnames).

e2e m5-t1 (Docker): browser registers as 170-web and has two-way audio
with a baresip phone both ways. The LAN and hotspot check on the owner's
server is still open (roadmap note).

Task: M5-T1

apps/web/package.json

@@ -17,6 +17,7 @@
     "react-dom": "^19.3.1",
     "react-hook-form": "^7.90.0",
     "react-router": "^8.4.0",
+    "sip.js": "0.21.2",
     "zod": "^4.6.7",
     "zustand": "^5.0.15"
   },