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/message-capture/messageMetadata.ts

@@ -515,7 +515,7 @@ export function renderDiscordMentions(
   content: string,
   metadata: string | null | undefined,
 ): string {
-  if (!content || !content.includes("<")) return content;
+  if (!content?.includes("<")) return content;
   const parsed = parseRichMessageMetadata(metadata);
   const roleName = new Map(
     (parsed?.mentionedRoles ?? []).map((r) => [r.id, r.name] as const),