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/app/bootstrap.ts

@@ -315,7 +315,7 @@ export async function initializeDiscordGateway() {
     gracefulShutdown("uncaughtException");
   });
 
-  process.on("unhandledRejection", (reason, promise) => {
+  process.on("unhandledRejection", (reason) => {
     const err =
       reason instanceof Error ? reason : new Error(String(reason ?? "unknown"));
     const code = (err as NodeJS.ErrnoException).code ?? "";