feat(element-calling): fix Element X discovery and harden federated calling

The element-calling feature only advertised the LiveKit focus via the
well-known org.matrix.msc4143.rtc_foci file, and relied on STUN
auto-detection for the public IP. Element X queries the MatrixRTC
transports registry endpoint and fails with MISSING_MATRIX_RTC_TRANSPORT
when it is absent, and blocked STUN egress silently left LiveKit
advertising a private IP (call connects but no video across servers).

- synapse: enable msc4143_enabled and advertise matrix_rtc.transports
  (MSC4519) with the site's element-calling URL, so Element X can
  discover the LiveKit focus instead of erroring out
- livekit: determine the public IP to advertise at runtime —
  explicit pin, then HTTPS egress detection (api.ipify.org /
  checkip.amazonaws.com / ifconfig.me), then STUN fallback with a
  warning; reject non-routable results (private/loopback/CGNAT)
- lk-jwt-service: append optional extra homeservers to
  LIVEKIT_FULL_ACCESS_HOMESERVERS via the new
  sovran_systemsOS.elementCalling.fullAccessHomeservers option
- add sovran_systemsOS.elementCalling.externalIP option to pin the
  advertised public IP for multi-WAN/VPN setups
- add element-calling-public-check.service: boot-time diagnostics for
  public DNS (via 1.1.1.1, bypassing local loopback overrides), JWT
  healthz through Caddy and via the public IP, and the transports
  endpoint — turns the silent -no media- failure into a visible error
- add restartTriggers so livekit/lk-jwt-service pick up regenerated
  runtime configs on rebuild

modules/core/roles.nix

@@ -79,6 +79,41 @@
       };
     };
 
+    # ── Element Calling (video/audio) tuning ──────────────────
+    elementCalling = {
+      fullAccessHomeservers = lib.mkOption {
+        type = lib.types.listOf lib.types.str;
+        default = [ ];
+        example = [ "matrix.peer.example.com" ];
+        description = ''
+          Additional Matrix server_names (beyond this server itself) that may
+          trigger LiveKit room creation on this server's SFU via lk-jwt-service.
+
+          Not needed for the common federated setup: each participant's client
+          always obtains its token from its own homeserver's JWT service and
+          publishes to its own SFU, and the participant who starts a call
+          creates the room on their own SFU — the remote user merely joins
+          (joining does not require full access).
+
+          Only set this for asymmetric cases: e.g. a peer homeserver that has
+          no focus of its own, or calls whose first participant lands on this
+          server's SFU but belongs to the peer.
+        '';
+      };
+      externalIP = lib.mkOption {
+        type = lib.types.nullOr lib.types.str;
+        default = null;
+        example = "203.0.113.10";
+        description = ''
+          Optional pin: force LiveKit to advertise this public IPv4 in its
+          host/TURN ICE candidates. Not required in normal operation — the
+          module auto-detects the public IP at runtime (HTTPS egress
+          detection, falling back to STUN). Set it only to override a
+          mis-detected address (e.g. multi-WAN/VPN setups).
+        '';
+      };
+    };
+
     # ── Domain setup registry ─────────────────────────────────
     domainRequirements = lib.mkOption {
       type = lib.types.listOf (lib.types.submodule {