perf(ai): kontiguitas batch budget + max_tokens dinamis + urutan kronologis RETURNING

- pickBatchWithinBudget: stop di overflow pertama (break), bukan skip —
  batch tetap prefix kronologis tanpa gap analisis di tengah timeline.
  Diekstrak ke batchBudget.ts (pure, estimator di-inject) + regression test.
- callModerationLLM: param opsional maxTokens; text/media caller menghitung
  ceiling dari estimasi prompt (floor 2048, cap 16384) — batch kecil tak
  lagi reserve window completion 16k.
- getPending/IncompleteMessagesByConversation: sort hasil UPDATE..RETURNING
  by created_at ASC — Postgres tak menjamin urutan, konsumen (anchor konteks
  messages[0], prefix batch) bergantung pada urutan kronologis.

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

@@ -0,0 +1,45 @@
+/**
+ * batchBudget.ts
+ *
+ * Pure batch-sizing helper extracted from batchProcessor.ts so it can be
+ * unit-tested without pulling in the Piscina worker pool, message store,
+ * or any other side-effectful import chain.
+ */
+import type { MessageRecord } from "../message-capture/types.js";
+
+/** Token estimator contract (satisfied by conversationContext.estimateTokens). */
+export type TokenEstimator = (text: string) => number;
+
+/**
+ * Picks a batch of messages within a token budget.
+ * `tokensPerMessage` accounts for JSON structure overhead around each entry.
+ * The estimator is injected so this stays a pure function — callers in the
+ * batch pipeline pass the tiktoken-based estimateTokens.
+ */
+export function pickBatchWithinBudget(
+  messages: MessageRecord[],
+  maxTokens: number,
+  tokensPerMessage: number,
+  estimateTokens: TokenEstimator,
+): MessageRecord[] {
+  const batch: MessageRecord[] = [];
+  let usedTokens = 0;
+
+  for (const msg of messages) {
+    const content = msg.edited_content ?? msg.content;
+    const msgTokens = estimateTokens(content) + tokensPerMessage;
+
+    // Stop at the first overflow instead of skipping: input is ordered
+    // created_at ASC, so a contiguous chronological prefix keeps the batch
+    // gap-free. Skipped-over messages would leave unanalyzed holes mid-
+    // timeline; anything past the budget is picked up by the next wave
+    // (processBatch always re-schedules after success).
+    if (usedTokens + msgTokens > maxTokens) {
+      break;
+    }
+    batch.push(msg);
+    usedTokens += msgTokens;
+  }
+
+  return batch;
+}