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/media_mcp.py

@@ -7,7 +7,7 @@ server and reports playback back, so voice controls and the web panel stay
 in sync.
 
 Per-session environment (set by the agent when spawning this MCP):
-- MEDIA_SERVER_URL  e.g. http://127.0.0.1:8092
+- MEDIA_SERVER_URL  e.g. http://127.0.0.1:8093
 - MEDIA_ROOM        LiveKit room name this session controls
 
 The "stop listening while music plays" rule lives in the agent itself: the
@@ -22,7 +22,7 @@ import os
 import httpx
 from mcp.server.fastmcp import FastMCP
 
-MEDIA_SERVER_URL = os.environ.get("MEDIA_SERVER_URL", "http://127.0.0.1:8092").rstrip("/")
+MEDIA_SERVER_URL = os.environ.get("MEDIA_SERVER_URL", "http://127.0.0.1:8093").rstrip("/")
 MEDIA_ROOM = os.environ.get("MEDIA_ROOM", "default")
 
 mcp = FastMCP("media")