feat: activate mod_xml_curl with b2bcall-fs-config (agente.md secao 26)

- apps/freeswitch-config (b2bcall-fs-config): Fastify service implementing
  the mod_xml_curl HTTP protocol (form-encoded POST -> XML response),
  containerized, no host port published
- reactivated mod_xml_curl in FreeSWITCH, binding restricted to
  directory|dialplan only (configuration was removed after testing showed
  it firing several unnecessary HTTP round-trips at boot for module
  configs we don't need dynamic — matches agente.md's own 'don't put every
  critical config through XML Curl' guidance)
- no extensions/dialplan tables exist yet (next phases), so the service
  always answers 'not found' for now — this phase only proves the wire
  protocol works without breaking the static vanilla config fallback
- verified end-to-end: user/8888 (nowhere) -> SUBSCRIBER_ABSENT via
  fs-config; user/1000 (static vanilla extension) -> USER_NOT_REGISTERED,
  proving FreeSWITCH correctly falls through to static XML when xml_curl
  says not found
- docs/XML_CURL.md, including the not-yet-authenticated endpoint note (fine
  while it only returns not-found; needs gateway-credentials before serving
  real directory/dialplan data)

infrastructure/freeswitch/Dockerfile

@@ -45,6 +45,7 @@ RUN --mount=type=secret,id=freeswitch_pat,required=true \
 COPY overrides/autoload_configs/modules.conf.xml /etc/freeswitch/autoload_configs/modules.conf.xml
 COPY overrides/autoload_configs/event_socket.conf.xml /etc/freeswitch/autoload_configs/event_socket.conf.xml
 COPY overrides/autoload_configs/acl.conf.xml /etc/freeswitch/autoload_configs/acl.conf.xml
+COPY overrides/autoload_configs/xml_curl.conf.xml /etc/freeswitch/autoload_configs/xml_curl.conf.xml
 COPY entrypoint.sh /usr/local/bin/entrypoint.sh
 RUN chmod +x /usr/local/bin/entrypoint.sh