discord_radio: desired-state streams, logout on release, no mention controls (node-26#25)

- Rewritten as RadioManager reconciling C2's retained
  nodes/{id}/discord/desired (token ids) and publishing
  nodes/{id}/discord/actual {applied_seq, sessions, streams}.
- One gateway session per leased token, one voice connection per
  (token, guild); a token no stream needs is logged OUT (client.close()),
  which is what lets C2 free it safely (item 1).
- Watchdog is a separate reconcile loop; recovering a dead session can no
  longer cancel it (item 2). Presence kept across rejoins.
- No message-content intent and no @bot leave/join mention controls (item 3).
- Local dashboard: /api/discord/join returns 409 with an explanation instead
  of a TypeError 500; /api/discord/leave pauses locally; GET /api/discord
  shows state (item 4). Join/leave results are reported to C2 (item 5).
- Token strings fetched over HTTPS from C2 (lease holder only), memory only.
- MQTT-loss fail-safe: logs every bot out after
  DISCORD_MQTT_LOSS_STOP_SECONDS (300, below C2's 600s lease grace).
- Legacy discord_join/discord_leave MQTT commands are ignored.
- Audio path unchanged: FFmpeg -f pulse on settings.pulse_source per
  connection, never Icecast.

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

drb-edge-node/app/routers/api.py

@@ -400,15 +400,25 @@ async def set_sdr(body: SdrSettingsBody):
     return {"ok": True, "running": running}
 
 
+@router.get("/discord")
+async def discord_status():
+    """What the node's radio bots are doing (desired state from C2, actual here)."""
+    return radio_bot.snapshot()
+
+
 @router.post("/discord/join")
-async def discord_join(guild_id: int, channel_id: int):
-    ok = await radio_bot.join(guild_id, channel_id)
-    if not ok:
-        raise HTTPException(500, "Failed to join voice channel")
-    return {"ok": True}
+async def discord_join():
+    # node-26#25 item 4: this used to call join() without a token and 500 every
+    # time. A node cannot lease a radio bot by itself — C2 owns the pool.
+    raise HTTPException(
+        409,
+        "Discord radio channels are started from Discord (/join) or the DRB web app; "
+        "the node cannot pick a radio bot on its own.",
+    )
 
 
 @router.post("/discord/leave")
 async def discord_leave():
-    await radio_bot.leave()
+    """Local stop: log every radio bot out until DRB sends a new channel list."""
+    await radio_bot.pause_locally()
     return {"ok": True}