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) ?? [];

services/discord-gateway/src/modules/ai-moderation/moderationOrchestrator.ts

@@ -194,7 +194,7 @@ export async function runModerationAnalysis(
       if (embeddings && embeddings.length === texts.length) {
         // index-aligned with semanticCandidates
         for (let i = 0; i < semanticCandidates.length; i++) {
-          const { target, cacheKey } = semanticCandidates[i];
+          const { cacheKey } = semanticCandidates[i];
           embeddingsByKey.set(cacheKey, embeddings[i]);
         }
 

services/discord-gateway/src/modules/ai-moderation/textBatchProcessor.ts

@@ -249,7 +249,8 @@ export async function runTextOnlyBatch(
           if (pics.length === 0) return { id: msg.id, lines: [] as string[] };
           const lines = await Promise.all(
             pics.map(async (url) => {
-              const img = urlImages.get(url)!;
+              const img = urlImages.get(url);
+              if (!img) return null;
               try {
                 const { data: resizedBuffer, mimeType: resizedMime } =
                   await resizeImageForVision(img.data, maxDim);

services/discord-gateway/src/modules/ai-moderation/textCacheStore.ts

@@ -3,7 +3,6 @@ import { createChildLogger } from "@/shared/logger/index";
 import { executeAll, executeGet } from "../../shared/database/drizzle.js";
 import { findBestEmbeddingMatch } from "./embeddingClient.js";
 import {
-  deleteExpiredQdrantPoints,
   deleteQdrantPoint,
   deleteQdrantPointsByContentHash,
   isQdrantConfigured,