perf(ai-moderation): compact system prompt + memoize build + hoist vision pass

- Memoize buildSystemPrompt by (mode|channelCulture); identical signatures
  now reuse the ~5k-token core instead of rebuilding per sub-batch call
  (textBatchProcessor rebuilt it inside the loop; a 200-msg batch re-sent
  the full system prompt ~4x). Correction tail stays per-attempt (uncached).
- Hoist URL-image -> vision evidence out of the per-sub-batch loop in
  textBatchProcessor: it depends only on fetched images + full target set,
  so compute once per whole batch, not per sub-batch.
- Compact system instructions: collapse 3x-duplicated 'evaluate by content
  alone' statements into one standalone rule; trim output.ts channel-culture
  + context framing already covered by rules.ts/system.ts; drop duplicate
  programming-error-log few-shot (id 17, covered by rules AMAN list).
- Fix misleading config default: AI_LLM_BASE_URL default -> omniroute
  (gateway already runs omniroute via BWS; 9router was dead/misleading).

typecheck + lint + build green.

services/discord-gateway/src/shared/config/index.ts

@@ -149,7 +149,7 @@ export const configSchema = z
     AI_LLM_BASE_URL: z
       .string()
       .url()
-      .default("https://9router.asepharyana.my.id/v1"),
+      .default("https://omniroute.imrnes.team/v1"),
     AI_LLM_MODEL: z.string().default("text"),
     // Vision uses the SAME router/base URL as text moderation
     // (AI_LLM_BASE_URL) but a different model alias. The dedicated NVIDIA