feat(gmw): route all LLM traffic through 9router

GMW moves off omniroute (100.121.180.82:20128) and off the direct NVIDIA
vision endpoint onto 9router, which runs on the same host as both services
(127.0.0.1:4014) — loopback avoids the TLS/proxy hop and localhost calls
bypass 9router's remote-key guard.

- gateway + backend: AI_LLM_BASE_URL default -> http://127.0.0.1:4014/v1
- drop stale 'omniroute' router references from comments/docs now that the
  active router is 9router (llmClient, llmCaller, ARCHITECTURE, AGENTS)

Verified against 9router before wiring: model 'text' -> gemini-3.5-flash-lite
(SSE, as the pipeline expects), 'multimodal' -> nemotron-3-nano-omni answers
image input, and gemini/gemini-embedding-001 returns 3072 dims — matching the
existing Qdrant collections (no reindex needed). The GMW key is already
registered in 9router's apiKeys table.

typecheck + lint + tests green (gateway 138, backend 37 excluding e2e).

services/discord-gateway/ARCHITECTURE.md

@@ -194,5 +194,5 @@ pipeline gauges registered by `app/metrics-collector.ts` —
   numeric snowflake IDs never trigger false positives.
 - **Semantic cache is batched** (one embed call + one Qdrant batch search),
   not N sequential round-trips. `ensureQdrantCollection` is memoized.
-- **Streaming is mandatory** against the omniroute base URL (non-stream waits for
+- **Streaming is mandatory** against the router base URL (non-stream waits for
   the full body and times out). `llmClient` aggregates SSE chunks.