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/digest/digest.go

@@ -154,15 +154,16 @@ func (g *Gate) Release() {
 }
 
 type Engine struct {
-	ChatURL  string
-	EmbedURL string
-	Model    string
-	Thinking bool
-	Client   *http.Client
-	Gate     *Gate
-	Timeout  time.Duration
-	Log      *slog.Logger
-	Cache    *dcache.DB
+	ChatURL    string
+	EmbedURL   string
+	EmbedModel string
+	Model      string
+	Thinking   bool
+	Client     *http.Client
+	Gate       *Gate
+	Timeout    time.Duration
+	Log        *slog.Logger
+	Cache      *dcache.DB
 
 	mu       sync.Mutex
 	Calls    uint64
@@ -201,6 +202,13 @@ func (e *Engine) model() string {
 	return "ornith-1.5-9b-uncensored"
 }
 
+func (e *Engine) embedModel() string {
+	if e != nil && e.EmbedModel != "" {
+		return e.EmbedModel
+	}
+	return "honcho-embed"
+}
+
 func (e *Engine) noteFallback(reason string) {
 	if e == nil {
 		return
@@ -379,7 +387,7 @@ func (e *Engine) embed(ctx context.Context, inputs []string) ([][]float64, error
 }
 
 func (e *Engine) embedBatch(ctx context.Context, inputs []string) ([][]float64, error) {
-	payload, _ := json.Marshal(map[string]any{"input": inputs, "model": "qwen3-embedding-0.6b"})
+	payload, _ := json.Marshal(map[string]any{"input": inputs, "model": e.embedModel()})
 	req, err := http.NewRequestWithContext(ctx, http.MethodPost, e.EmbedURL, bytes.NewReader(payload))
 	if err != nil {
 		return nil, err
@@ -541,6 +549,9 @@ func countBudget(chunks []string, budget int) int {
 	return n
 }
 
+// CapBudget trims s to the digest input budget (SmallDoc estimated tokens).
+func CapBudget(s string) string { return trimTokens(s, SmallDoc) }
+
 func trimTokens(s string, max int) string {
 	r := []rune(s)
 	n := max * 4