Discord: log node-reported stream state; remove a radio channel or a server

- handle_actual logs every node-reported state TRANSITION (not heartbeats)
  with stream/node/guild/channel/token ids and the node's error text: info
  for connected/connecting/stopped, warning for error, and a dedicated
  "radio bot X failed to log in on node N: <reason>" line for login_failed.
  The node's reason code is now stored in actual.code and shown next to the
  error on Settings > Discord and the node page.
- Remove a radio channel permanently: streams.delete_stream stops it through
  desired state first (node republished without it, token -> releasing per
  #216, freed on the node's ack), then deletes the doc in a transaction that
  refuses if the channel was restarted in between. DELETE
  /discord/streams/{id} (platform admin) and DELETE
  /discord/nodes/{node}/streams/{id} (admin or org owner, same org + node
  only; operators can still stop, not remove). Audited.
- Remove a server: DELETE /discord/guilds/{id} (admin) deletes every radio
  channel in it the same way, then tombstones discord_guilds (org_id None +
  unlinked_at, #218 semantics, plus removed_at) so it never falls back to
  DISCORD_UNLINKED_GUILD_ORG_ID; the admin page hides it, and /drb link or the
  admin link brings it back. Optional leave_radio_bots makes pool bots and the
  server's org's own BYO bots leave via Discord REST; the DRB server bot stays
  until kicked, and the UI says so. Audited.
- UI: Remove (with confirm) on Radio channels, Servers and the node page list.

Refs #213, #221

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

drb-c2-core/tests/fake_firestore.py

@@ -75,6 +75,9 @@ class _Txn:
     def update(self, ref: _DocRef, data: dict):
         self.writes.append(("update", ref.coll, ref.id, copy.deepcopy(data), True))
 
+    def delete(self, ref: _DocRef):
+        self.writes.append(("delete", ref.coll, ref.id, {}, False))
+
 
 class FakeFirestore:
     def __init__(self):
@@ -91,6 +94,10 @@ class FakeFirestore:
 
     def _write(self, kind: str, coll: str, doc_id: str, data: dict, merge: bool):
         docs = self.data.setdefault(coll, {})
+        if kind == "delete":
+            docs.pop(doc_id, None)
+            self.versions[(coll, doc_id)] = self.versions.get((coll, doc_id), 0) + 1
+            return
         if kind == "update" and doc_id not in docs:
             raise KeyError(f"No document to update: {coll}/{doc_id}")
         if merge and doc_id in docs: