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/api/api_test.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	"git.wntrmute.dev/kyle/sift/internal/auth"
+	"git.wntrmute.dev/kyle/sift/internal/digest"
 	"git.wntrmute.dev/kyle/sift/internal/fetch"
 	"git.wntrmute.dev/kyle/sift/internal/guard"
 )
@@ -156,6 +157,18 @@ func TestDigestFallbackWhenUnset(t *testing.T) {
 	}
 }
 
+func TestCapDigestFallback(t *testing.T) {
+	big := strings.Repeat("word ", 50000)
+	out, warn := capDigestFallback(big, nil)
+	if digest.EstTokens(out) > digest.SmallDoc {
+		t.Fatalf("tokens %d", digest.EstTokens(out))
+	}
+	joined := strings.Join(warn, " ")
+	if !strings.Contains(joined, "digest budget") {
+		t.Fatalf("warn %v", warn)
+	}
+}
+
 func TestNever502(t *testing.T) {
 	s, key := setupAPI(t, nil)
 	rec := postJSON(t, s, "/v2/scrape", key, map[string]string{"url": "http://127.0.0.1/"})