p2.1: honcho-embed, wayback http/204, memento, keys owner

Embed requests send dixie.embed_model (default honcho-embed).
Wayback accepts http://web.archive.org, rewrites to https, skips
non-200 closest.status. Memento-Datetime is parsed as an HTTP date
and future/garbage values become empty snapshot_at. keys add writes
via temp+fchown/chmod/rename so root does not clobber keys.toml.
Digest fallback is capped at the 6k-token budget.

cmd/sift/serve.go

@@ -106,13 +106,14 @@ func serveMain() {
 			log.Warn("digest cache", "err", err)
 		}
 		srv.Digest = &digest.Engine{
-			ChatURL:  cfg.Dixie.Chat,
-			EmbedURL: cfg.Dixie.Embed,
-			Model:    cfg.Digest.Model,
-			Thinking: cfg.Digest.Thinking,
-			Gate:     digest.NewGate(cfg.Dixie.MaxInflight, cfg.Dixie.QueueDepth, config.ParseDuration(cfg.Dixie.QueueWait, digest.DefaultQWait)),
-			Cache:    cache,
-			Log:      log,
+			ChatURL:    cfg.Dixie.Chat,
+			EmbedURL:   cfg.Dixie.Embed,
+			EmbedModel: cfg.Dixie.EmbedModel,
+			Model:      cfg.Digest.Model,
+			Thinking:   cfg.Digest.Thinking,
+			Gate:       digest.NewGate(cfg.Dixie.MaxInflight, cfg.Dixie.QueueDepth, config.ParseDuration(cfg.Dixie.QueueWait, digest.DefaultQWait)),
+			Cache:      cache,
+			Log:        log,
 		}
 	}
 	if cfg.Context.Honcho != "" {