fix(voice): resolve Redis subscriber corruption, transmitter races, and DB error logging

Redis root cause: VoiceHandler passed redisPub to transmitter.start(),
which called .subscribe() on it — permanently converting the publish
connection to subscriber mode. Every subsequent command reply and status
update failed.

Fixes:
1. Transmitter now creates its own Redis client via new IORedis()
2. Mutual exclusion gate serialises start/stop to prevent null-deref races
3. PassThrough drain listeners cleaned up to stop MaxListenersExceeded
4. FFmpeg SIGTERM flagged as expected exit (no more false level-50 errors)
5. Voice recording repo captures PG error code/detail for diagnostics

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

services/discord-gateway/src/shared/database/voiceRecordingRepo.ts

@@ -40,19 +40,13 @@ export async function insertVoiceRecording(
       .values(recording)
       .onConflictDoNothing();
   } catch (error) {
-    const detail =
-      error instanceof Error
-        ? {
-            message: error.message,
-            name: error.name,
-            ...((error as Record<string, unknown>).code !== undefined
-              ? { code: (error as Record<string, unknown>).code }
-              : {}),
-            ...((error as Record<string, unknown>).detail !== undefined
-              ? { detail: (error as Record<string, unknown>).detail }
-              : {}),
-          }
-        : String(error);
+    const err = error as Record<string, unknown>;
+    const detail: Record<string, unknown> = {
+      message: error instanceof Error ? error.message : String(error),
+      name: error instanceof Error ? error.name : undefined,
+    };
+    if (err.code !== undefined) detail.code = err.code;
+    if (err.detail !== undefined) detail.detail = err.detail;
     logger.error(
       { id: recording.id, error: detail },
       "Failed to insert voice recording",