hindsight: cap RETAIN_MAX_COMPLETION_TOKENS for vLLM's 8192 ctx

Default 64000 exceeded vLLM Qwen2.5-32B-Instruct-AWQ's --max-model-len
8192, causing every retain call to 500 with
'max_tokens=64000 cannot be greater than max_model_len=8192'.
llama-swap's Qwen3.8-27B ran at ctx=65536 so this never surfaced
before the vLLM cutover. Lowered to 4096.

cluster/applications/hindsight/values.yaml

@@ -73,6 +73,16 @@ api:
     HINDSIGHT_API_LLM_BASE_URL: "http://astro-orbiter:8000/v1"
     HINDSIGHT_API_LLM_PROVIDER: "openai"
     HINDSIGHT_API_LLM_MODEL: "Qwen2.5-32B-Instruct-AWQ"
+    # vLLM's Qwen2.5-32B-Instruct-AWQ is deployed with --max-model-len 8192
+    # (roles/deploy-vllm/defaults/main.yml vllm_max_model_len; llama-swap's
+    # Qwen3.8-27B ran at ctx=65536, matching Hindsight's default
+    # RETAIN_MAX_COMPLETION_TOKENS=64000). Discovered via a live retain 500:
+    # "max_tokens=64000 cannot be greater than max_model_len=8192" — every
+    # retain call was requesting a 64K-token completion budget the new
+    # backend physically cannot serve. Lowered to 4096, comfortably under
+    # 8192 with room for the ~3000-char (RETAIN_CHUNK_SIZE) input chunk plus
+    # prompt overhead. Verified with a live hindsight_retain call post-fix.
+    HINDSIGHT_API_RETAIN_MAX_COMPLETION_TOKENS: "4096"
     # DO NOT set HINDSIGHT_API_EMBEDDINGS_* here (t_e6facb19, 2026-08-31
     # attempted this, reverted after a production incident — see below).
     #