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/overrides/autoload_configs/xml_curl.conf.xml

@@ -0,0 +1,11 @@
+<configuration name="xml_curl.conf" description="cURL XML Gateway">
+  <bindings>
+    <binding name="b2bcall-fs-config">
+      <!-- b2bcall-fs-config ainda so responde "not found" pra tudo (nao
+           existe extensions/dialplan persistidos ainda — fases seguintes).
+           A config estatica vanilla continua valendo como fallback. -->
+      <param name="gateway-url" value="http://fs-config:8080/" bindings="directory|dialplan"/>
+      <param name="timeout" value="5"/>
+    </binding>
+  </bindings>
+</configuration>