fix: move media server to 8093 — YOURLS/Apache owns 8092 on xNAS

The Unraid deployment's /media/api requests were answered by an
Apache/YOURLS stack squatting on 8092, surfacing as UI 503s; the CI
healthcheck (curl without -f) counted that 503 as healthy, so run #15
deployed green against a broken media server.

- media server port 8092 -> 8093 everywhere (server, MCP, agent, nginx,
  compose, CI, .env.example, docs, tests).
- healthchecks use curl -f in compose and rebuild.yml so HTTP error
  statuses fail deployment.
- tests default to the Unraid deployment (VOICE_WEB_URL=
  https://192.168.86.2:9178); token goes through nginx (loopback :8091
  is unreachable off-host); bind-mount-coupled tests skip on remote
  targets. AGENTS.md documents the convention and both failure modes.

nginx.conf

@@ -27,7 +27,7 @@ server {
     # external live-stream URLs (http:// sources would be blocked as mixed
     # content in the browser, so they are re-served over HTTPS from here).
     location /media/api/ {
-        proxy_pass http://127.0.0.1:8092/media/api/;
+        proxy_pass http://127.0.0.1:8093/media/api/;
         proxy_buffering off;
         proxy_read_timeout 3600s;
         proxy_send_timeout 3600s;