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/ai-moderation/mediaDownloader.ts

@@ -450,7 +450,7 @@ export async function downloadMediaCandidate(
   if (candidate.customEmojiId || candidate.stickerName) {
     const vck = candidate.customEmojiId
       ? makeCustomEmojiCacheKey(candidate.customEmojiId)
-      : makeStickerCacheKey(candidate.stickerName!);
+      : makeStickerCacheKey(candidate.stickerName ?? "");
     const cached = await getCachedMediaAnalysis(vck);
     if (cached) {
       const existing = mediaAnalysisMap.get(targetId) ?? [];