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/backend/src/shared/utils/index.ts

@@ -108,5 +108,5 @@ export async function retryWithBackoff<T>(
       });
     }
   }
-  throw lastError!;
+  throw lastError ?? new Error("Request failed after all retries");
 }