docs: media player subsystem architecture and conventions

Document the media server process, /media/ port map, library layout on
the ./media bind mount, Hope's media MCP toolset, the music/mic
mutual-exclusion rule, and yt-dlp download/proxy conventions.

media/jobs/477f760d9615.json

@@ -0,0 +1 @@
+{"id": "477f760d9615", "track_id": "qATbf5-D8bU", "stage": "ready", "progress": 100.0, "error": null, "started_at": 1788023839.0112658}

media/jobs/7436101dd688.json

@@ -0,0 +1 @@
+{"id": "7436101dd688", "track_id": "LWtRNwbmlb4", "stage": "ready", "progress": 100.0, "error": null, "started_at": 1788023614.9862244}

media/jobs/d1aab72f3953.json

@@ -0,0 +1 @@
+{"id": "d1aab72f3953", "track_id": "hR7xhDhDGR0", "stage": "error", "progress": 0.0, "error": "ERROR: [youtube] hR7xhDhDGR0: This video is unavailable", "started_at": 1788023758.6851313}

media/library.json

@@ -0,0 +1 @@
+{"tracks": {}, "version": 42}