chore(comms): retire remaining dead channel residue

Completes the reference cleanup with the genuinely-dead comms bits (the live
extraction pipeline + Campaign.channels marketing field are kept):

- models/audit.py: remove the dead CHANNEL_ACCESS_DENIED enum member (str-backed
  column, no migration; its do_server/flow_server map keys were removed in 925e00d4)
- panel stream.ts: remove the dead getChannelPermissions method (no matching
  backend route — routes/stream.py serves only GET /permissions). Kept the
  ChannelPermissions interface (still used by the live getPermissions) and every
  other streamApi method (submit/complete/extract/stats/permissions hit live endpoints)
- api/schemas/stream.py: reword stale 'channel' field descriptions on live
  extraction fields (Target stream / Group within stream)
- panel knowledge-base.ts: fix a mock-mode example source string

No regressions: only provably-dead code retired; extraction pipeline untouched.

panel/src/lib/api/knowledge-base.ts

@@ -64,7 +64,7 @@ const mockSearchResults: KBSearchResult[] = [
   {
     content:
       "BE-DEV-1: I've completed the task validation logic. The acceptance criteria now require at least one item before a task can be created.",
-    source: "channel:backend/session:abc123",
+    source: "journal:be-dev-1",
     score: 0.75,
     index_type: KBIndexType.CONVERSATIONS,
     metadata: { agent: "be-dev-1", timestamp: "2024-01-15T10:30:00Z" },

panel/src/lib/api/stream.ts

@@ -164,23 +164,4 @@ export const streamApi = {
     const { data } = await api.get<ChannelPermissions[]>("/stream/permissions");
     return data;
   },
-
-  /**
-   * Get permissions for a specific channel
-   */
-  getChannelPermissions: async (
-    channelName: string,
-  ): Promise<ChannelPermissions> => {
-    if (isMockMode()) {
-      return {
-        channel_name: channelName,
-        can_transcribe: true,
-        can_extract: true,
-      };
-    }
-    const { data } = await api.get<ChannelPermissions>(
-      `/stream/permissions/channel/${channelName}`,
-    );
-    return data;
-  },
 };