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.

internal/config/config.go

@@ -21,6 +21,7 @@ type File struct {
 type Dixie struct {
 	Chat        string `toml:"chat"`
 	Embed       string `toml:"embed"`
+	EmbedModel  string `toml:"embed_model"`
 	MaxInflight int    `toml:"max_inflight"`
 	QueueDepth  int    `toml:"queue_depth"`
 	QueueWait   string `toml:"queue_wait"`