discord_radio: presence follows voice; name the reason a voice join fails

Live symptom: the radio bot shows "Listening to <system>" but is not in the
voice channel.

- Presence was set once on the first successful join and never cleared, so a
  bot whose voice dropped (or whose rejoin kept failing) still looked healthy.
  Presence is now synced at the end of every reconcile: set only while a voice
  connection for that bot is up, cleared otherwise.
- Pre-check the bot's permissions (View Channel, Connect, Speak) and the
  channel's user limit before joining. Discord silently ignores a join the bot
  may not make; discord.py then times out after 30s and the admin page read
  "Could not connect to the voice channel (TimeoutError: )". Now: code
  missing_permissions / channel_full with the missing permission named.
- A connect timeout gets its own code (connect_timeout) and readable text.
- requirements: discord.py[voice]>=2.7 — Discord has required DAVE E2EE voice
  since 2026-03-01 and 2.7 is the first discord.py with it (the extra pulls
  davey). Today's resolution was already 2.7.1 + davey 0.1.6; this stops an
  older resolution from ever being accepted.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

drb-edge-node/requirements.txt

@@ -3,7 +3,9 @@ uvicorn[standard]
 pydantic-settings
 paho-mqtt>=2.0.0
 httpx
-discord.py[voice]
+# >=2.7: Discord requires DAVE (E2EE) voice since 2026-03-01; 2.7 is the first
+# discord.py with it (the [voice] extra pulls `davey`). Older releases cannot join voice.
+discord.py[voice]>=2.7
 PyNaCl
 python-multipart
 pytest