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.

.env.example

@@ -34,7 +34,7 @@ VOICE_DEFAULT_USER=shane
 #   media/files/  transcoded mp3 tracks
 #   media/thumbs/ cover art
 #   media/jobs/   download job status
-# MEDIA_SERVER_PORT=8092
+# MEDIA_SERVER_PORT=8093 (8092 is taken by YOURLS/Apache on xNAS)
 #
 # Optional: export cookies.txt from a browser logged into YouTube and drop it
 # in ./media/cookies.txt to avoid "confirm you're not a bot" download blocks.