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/llmCaller.ts

@@ -58,6 +58,9 @@ export async function callModerationLLM(
   // callers pass a prompt-derived ceiling so small batches don't reserve a
   // 16k completion budget (some routers pre-allocate KV cache per max_tokens).
   maxTokens?: number,
+  // Concurrency lane: "text" (default) uses AI_LLM_MAX_CONCURRENT; "media"
+  // uses AI_LLM_MEDIA_MAX_CONCURRENT.
+  lane: "text" | "media" = "text",
 ): Promise<{
   results: AnalysisResult[];
   raw: ChatCompletion | null;
@@ -96,6 +99,7 @@ export async function callModerationLLM(
             // consumes chunks incrementally — timeout only fires on a real
             // stall. llmClient aggregates the stream into a ChatCompletion.
             stream: true,
+            lane,
           });
 
           if (!completion)