feat(litellm): load-share embed/rerank across esh-ml1 + nh3-ml1 with intra-group failover

- qwen3-embedding and reranker: second deployment on nh3-ml1 (config);
  reranker-a3-bge-v2-m3: second DB deployment via /model/new.
- router_settings.enable_weighted_failover: true. Only the three
  multi-deployment groups are affected.
- Measured by stopping nh3-ml1 TEI: without failover 7/40 embeds 500'd;
  with it rerank 80/80, embed 38/40 at onset and 60/60 over a 34 s outage;
  nh3 rejoins rotation after restart. Split 12/28 embed, 22/18 rerank.

stacks/litellm/conf/config.yaml

@@ -443,15 +443,14 @@ model_list:
   #     explicitly. Operator ruling 2026-08-23. ---
 
   # --- Qwen3 embeddings ---
-  # ONE backend: esh-ml1 (RTX 2000E Ada), served by Hugging Face TEI — the fleet's
-  # embed/rerank engine from 2026-09-25 (Prime). fv-ml1's vLLM seat was retired the
-  # same day. TEI's /v1/embeddings is OpenAI-compatible, so the provider stays
-  # `hosted_vllm/`. Same model as before; TEI vectors match the old vLLM ones
-  # (cosine median 0.999925, inside vLLM's own noise; old-index retrieval overlap
-  # 0.988 vs 0.986 self-noise), so no index needed re-embedding.
-  # ⚠ Single backend until the second RTX 2000 lands (Prime, 2026-09-25): if
-  # esh-ml1, esh-pve or the ESH mesh route is down, fleet embeddings are down.
-  # ⚠ Never point this name at a DIFFERENT embedding model — vectors are
+  # TWO backends, load-shared (simple-shuffle), since 2026-09-26 (Prime): esh-ml1
+  # (ESH) and nh3-ml1 (NH3). Both are an RTX 2000E Ada running Hugging Face TEI
+  # 89-1.9.4 with the same model. Parity measured 2026-09-25: cosine esh vs nh3
+  # min 0.999993 = each host's own floor; overlap@10 1.000
+  # (servers/nh3-ml1/README.md). TEI's /v1/embeddings is OpenAI-compatible, so the
+  # provider is `hosted_vllm/`. TEI vectors match the retired vLLM seats (cosine
+  # median 0.999925), so no index needed re-embedding.
+  # ⚠ Never point this name at a DIFFERENT embedding model: vectors are
   # model-specific. Engine bake-off: docs/pfi/embed-rerank-tei-vs-vllm-bakeoff.md.
   - model_name: qwen3-embedding
     litellm_params:
@@ -460,6 +459,13 @@ model_list:
       api_key: os.environ/VLLM_API_KEY
     model_info:
       mode: embedding
+  - model_name: qwen3-embedding
+    litellm_params:
+      model: hosted_vllm/Qwen/Qwen3-Embedding-0.6B
+      api_base: http://10.100.50.80:8001/v1
+      api_key: os.environ/VLLM_API_KEY
+    model_info:
+      mode: embedding
 
   # --- qwen3-reranker RETIRED 2026-08-20. It named Qwen3-Reranker-0.6B on :8002,
   #     the incumbent the R43 bake-off replaced on 2026-08-06 after measuring it
@@ -498,12 +504,13 @@ model_list:
     model_info:
       mode: chat
   # reranker → generic capability name for rerank (bge-reranker-v2-m3 since the
-  # R43 cutover). ONE backend: TEI on esh-ml1 (fv-ml1's vLLM seat retired
-  # 2026-09-25). ⚠ TEI's /rerank takes `texts`, not `documents`: this MUST use the
+  # R43 cutover). TWO backends, load-shared, since 2026-09-26 (Prime): TEI on
+  # esh-ml1 and nh3-ml1. esh vs nh3 max score diff 0.0014, inside esh's own
+  # 0.0020 run-to-run floor; top-1 identical. The DB-only alias
+  # `reranker-a3-bge-v2-m3` carries the same two deployments.
+  # ⚠ TEI's /rerank takes `texts`, not `documents`: this MUST use the
   # `huggingface/` provider. `hosted_vllm/` gets a 422 "missing field texts".
   # api_base has NO /v1. api_key is a placeholder; TEI runs without auth.
-  # Parity vs the retired vLLM seat: top-1/top-3 identical (129/130 top-1 over
-  # two runs); scores move up to 0.019, reshuffling only near-zero tail docs.
   - model_name: reranker
     litellm_params:
       model: huggingface/BAAI/bge-reranker-v2-m3
@@ -511,6 +518,13 @@ model_list:
       api_key: "none"
     model_info:
       mode: rerank
+  - model_name: reranker
+    litellm_params:
+      model: huggingface/BAAI/bge-reranker-v2-m3
+      api_base: http://10.100.50.80:8013
+      api_key: "none"
+    model_info:
+      mode: rerank
 
   # --- lfm25-vl-3b → LiquidAI LFM2.5-VL-3B (Q5_K_M + mmproj Q8_0), llama.cpp server on
   #     nh3-ml1 :8030 (stacks/lfm-vl-seat, 2026-09-26). Image understanding for
@@ -1077,6 +1091,18 @@ general_settings:
       forward_headers: true
       include_subpath: true
 
+router_settings:
+  # Intra-group failover for simple-shuffle. When one deployment fails, the SAME
+  # request re-picks among the group's other healthy deployments, excluding the
+  # failed one. Without it, a dead backend in a 2-deployment group kept failing
+  # a share of requests: the router's default 2 retries re-picked at random and
+  # hit the dead one again. Measured 2026-09-26 with nh3-ml1 TEI stopped: 7/40
+  # embeds returned 500.
+  # Affects ONLY multi-deployment groups (qwen3-embedding, reranker,
+  # reranker-a3-bge-v2-m3 as of 2026-09-26). A single-deployment group has no
+  # other deployment to re-pick, so its behaviour is unchanged.
+  enable_weighted_failover: true
+
 litellm_settings:
   # vLLM rejects some OpenAI params other backends accept; drop silently
   # rather than 400 the caller.