Answers to similar questions, not only the same one

llmgw keys cache --similar 0.95 also gives a chat question the answer to
one like it: the last user message embedded by router.cache.embedding_model
(asked on the gateway's account, as a judge is, within 2 s), compared by
cosine with the questions answered in the same conversation — system
prompt, earlier turns, tools and sampling the same. The exact cache is
asked first; an exact hit embeds nothing. Said as x-llmgw-cache: similar
with x-llmgw-cache-similarity, charged as a hit.

The vectors live with their answers in the gateway's memory, counted in
max_bytes and evicted with them, not in a pgvector table as BACKLOG I1
planned: the cache keeps answer text out of the store on purpose. A
question over 2,000 characters is reused only when the same. Migration
0051 holds the setting. BACKLOG I1.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/reference.md

@@ -53,6 +53,7 @@ rather than repeated.
 | `retain_days` | `int` | [Batch](#batch) |
 | `max_bytes` | `int64` | [Cache](#cache) |
 | `max_entry_bytes` | `int64` | [Cache](#cache) |
+| `embedding_model` | `string` | [Cache](#cache) |
 | `model` | `string` | [Canary](#canary) |
 | `percent` | `int` | [Canary](#canary) |
 | `min_requests` | `int` | [Canary](#canary) |
@@ -769,6 +770,15 @@ Past it, the answers used longest ago go first.
 MaxEntryBytes bounds one answer; a longer one is not kept. 1 MB
 unless set.
 
+### `embedding_model`
+
+`string`
+
+EmbeddingModel embeds the questions of projects that reuse answers to
+similar ones (llmgw keys cache --similar): an embedding model in the
+catalog. Unset, those projects reuse only the same request's answer.
+The gateway asks it on its own account, as it asks a judge.
+
 ## Canary
 
 ### `model`