fix(gateway): separate text and media lanes in AI analysis queue (#86)

Image messages previously blocked the whole analysis pipeline:
- conversationProcessing was a single lock per conversation; processBatch
  awaited BOTH text and media worker jobs before releasing it, so a fast
  text verdict sat unused until the slow vision/media batch finished
- one global LLM semaphore (AI_LLM_MAX_CONCURRENT) was shared by text and
  media, so a vision backlog could starve text inference
- recovery worker gated on conversationProcessing.size

Now the queue is split into independent text/media lanes:
- conversationProcessing maps key -> Partial<Record<lane, startedAt>>;
  each lane holds its own lock and frees it the moment ITS worker job
  resolves (ownership-guarded clear prevents stale timers clearing newer
  slots)
- two LLM semaphores: AI_LLM_MAX_CONCURRENT (text, default 8) and
  AI_LLM_MEDIA_MAX_CONCURRENT (media, default 4) via
  withLlmConcurrency(fn, { lane })
- batchScheduler schedules per conversation+lane (timer keys
  '<key>::<lane>'); splitMessagesByLane/laneOfMessage moved to pure
  analysisLanes.ts (unit-testable without Piscina)
- ai-analysis-worker batch jobs carry a lane field; per-lane active
  request gauges (active_text_requests / active_media_requests)
- added tests/analysisLaneLock.test.ts (7 tests: independent lane locks,
  preserving other-lane lock, clear-all, ownership guard, lane split)

Docs: ARCHITECTURE.md + AGENTS.md concurrency model updated.
typecheck/lint/test(138)/build all green.

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

@@ -0,0 +1,35 @@
+/**
+ * analysisLanes.ts
+ *
+ * Pure lane helpers for the AI-analysis queue. Kept free of any import chain
+ * that pulls Piscina/worker/DB so they can be unit-tested in isolation (the
+ * scheduler's `splitMessagesByLane` used to live in batchScheduler.ts, which
+ * transitively imports the worker pool).
+ */
+import type { MessageRecord } from "../message-capture/types.js";
+import type { AnalysisLane } from "./conversationState.js";
+import { hasMediaContent } from "./mediaAnalysisClient.js";
+
+export type { AnalysisLane } from "./conversationState.js";
+
+/** True when this message belongs to the media lane (has attachment/sticker/embed). */
+export function laneOfMessage(message: MessageRecord): AnalysisLane {
+  return hasMediaContent(message) ? "media" : "text";
+}
+
+/**
+ * Splits an arbitrary message array into per-lane lists. Used when the
+ * scheduler runs a conversation-wide pass (lane omitted): each lane gets its
+ * own subset so text and media never share a worker job.
+ */
+export function splitMessagesByLane(messages: MessageRecord[]): {
+  text: MessageRecord[];
+  media: MessageRecord[];
+} {
+  const text: MessageRecord[] = [];
+  const media: MessageRecord[] = [];
+  for (const m of messages) {
+    (laneOfMessage(m) === "media" ? media : text).push(m);
+  }
+  return { text, media };
+}