Fix push-digest sentinel filter, watermark, and cleanups from review

- push digest queries now exclude _duplicate channel like every other
  visibility query (bookmarks list/count and NewClassifiedSince)
- advance push watermark to newest scanned story, not pass-start now, so
  stories arriving during a long send pass aren't re-counted next pass
- replace hand-rolled escapeHTMLText with stdlib html.EscapeString
- drop em-dashes from user-facing copy; bump PWA CACHE_VERSION so clients
  pick up the changed shell assets

internal/web/feed.go

@@ -3,6 +3,7 @@ package web
 import (
 	"encoding/json"
 	"encoding/xml"
+	"html"
 	"log/slog"
 	"net/http"
 	"strings"
@@ -284,20 +285,8 @@ func contentToHTML(text string) string {
 			continue
 		}
 		b.WriteString("<p>")
-		b.WriteString(strings.ReplaceAll(escapeHTMLText(para), "\n", "<br>"))
+		b.WriteString(strings.ReplaceAll(html.EscapeString(para), "\n", "<br>"))
 		b.WriteString("</p>")
 	}
 	return b.String()
 }
-
-// escapeHTMLText escapes the five characters that matter inside HTML text so the
-// article body can't inject markup into content:encoded.
-func escapeHTMLText(s string) string {
-	return strings.NewReplacer(
-		"&", "&amp;",
-		"<", "&lt;",
-		">", "&gt;",
-		`"`, "&#34;",
-		"'", "&#39;",
-	).Replace(s)
-}