feat: semantic (embedding-based) response cache

- cache.rs: cosine-similarity store (threshold, TTL, ring buffer), partitioned by
  routing signature; tracks hit rate and model cost saved
- pipeline: embed routing text, look up before the model call (route_source=cache on
  hit), insert successful responses; stats record cache hits
- config [cache] + admin settings card; dashboard shows hit-rate and savings
- embedding model resolved from the pool (or first kind="embedding")

config/gateway.example.toml

@@ -150,3 +150,15 @@ enabled   = true
 file      = "config/stats.jsonl"   # empty = in-memory only
 ring_size = 500                     # how many recent requests to keep in the feed
 retention = "7d"                    # aggregate/series depth (replayed on startup)
+
+# ============================================================================
+# Semantic cache — returns a cached answer for prompts semantically near a
+# previous one, skipping the model call. Requires an embedding model in the pool.
+# Disabled by default. (Cache settings take effect on restart.)
+# ============================================================================
+[cache]
+enabled     = false
+threshold   = 0.92                  # cosine similarity for a hit (0..1)
+ttl         = "1h"
+max_entries = 1000
+# embed_model = "local-embed"       # defaults to the first kind = "embedding" model