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.

agent/agent.py

@@ -688,7 +688,7 @@ def build_mcp_toolsets(user_profile: dict, task_dir: str) -> list[mcp.MCPToolset
 
     # Media tools talk to the local media server; the room name scopes player
     # state so concurrent sessions don't fight over one virtual player.
-    media_server_url = os.environ.get("MEDIA_SERVER_URL", "http://127.0.0.1:8092")
+    media_server_url = os.environ.get("MEDIA_SERVER_URL", "http://127.0.0.1:8093")
     media_mcp_script = os.path.join(os.path.dirname(os.path.abspath(__file__)), "media_mcp.py")
     toolsets.append(
         mcp.MCPToolset(