style(gateway,backend): clear all biome warnings (no warnings left behind)

Address every remaining biome lint/format warning across both services
so the codebase ships warning-free:

- textCacheStore: drop unused deleteExpiredQdrantPoints import; hash
  image cache key (sha256[:32]) so long/base64 URLs no longer blow the
  text_analysis_cache PK B-tree 8191-byte index (was aborting the media
  analysis lock INSERT).
- bootstrap: drop unused unhandledRejection promise param.
- moderationOrchestrator: drop unused  destructure at L197.
- mediaDownloader / textBatchProcessor / transmitter: replace non-null
  assertions with proper null guards (stickerName ?? '', urlImages.get
  guard, backpressureQueue.shift guard).
- backend utils: throw lastError ?? fallback instead of lastError!.
- message-capture: remove unused  (retentionDb),  (moderationActionsDb,
  reviewsDb); simplify renderDiscordMentions guard to optional chain.
- transmitter: remove dead write-only  field + its assignments.

No behavior change beyond the cache-key hashing (now deterministic
fixed-length) and the intentional null-safety guards.

services/discord-gateway/src/modules/voice-recording/transmitter.ts

@@ -27,8 +27,6 @@ export class VoiceTransmitter {
   private gate = Promise.resolve();
   /** Set true before sending SIGTERM so exit handler knows it's intentional */
   private _expectedExit = false;
-  /** True while the underlying stream is in a drain state (backpressure) */
-  private draining = false;
 
   /**
    * Start listening for PCM audio data from Redis and stream to Discord
@@ -179,15 +177,13 @@ export class VoiceTransmitter {
             const canContinue = stream.write(pcmBuffer);
             // Backpressure: queue until drain
             if (!canContinue) {
-              this.draining = true;
               stream.once("drain", () => {
-                this.draining = false;
-                // Re-acquire stream reference (could have been replaced by restart)
                 const currentStream = this.pcmStream;
                 if (!currentStream || !this.isActive) return;
                 // Flush queued chunks
                 while (this.backpressureQueue.length > 0) {
-                  const queued = this.backpressureQueue.shift()!;
+                  const queued = this.backpressureQueue.shift();
+                  if (!queued) break;
                   try {
                     if (!currentStream.write(queued)) break;
                   } catch (err) {
@@ -237,7 +233,6 @@ export class VoiceTransmitter {
       this.isActive = false;
 
       this.backpressureQueue = [];
-      this.draining = false;
 
       if (this.pcmStream) {
         this.pcmStream.removeAllListeners("drain");