feat(admin): per-feed Subscribe chips with copy/open/validate for RSS/Atom/JSON

Replace the stacked RSS/Atom URL rows in the dashboard with a compact
"Subscribe" chip block exposing all three feed formats — including JSON
Feed, previously absent from the admin UI. Each chip carries copy, open,
and validate actions; validation links to the W3C Feed Validator (RSS/Atom)
and validator.jsonfeed.org (JSON). The Table view's RSS+Atom columns fold
into a single Formats column.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/infrastructure/urls.ts

@@ -13,6 +13,10 @@ export function feedAtomUrl(feedId: string, env: Env): string {
   return `${baseUrl(env)}/atom/${feedId}`;
 }
 
+export function feedJsonUrl(feedId: string, env: Env): string {
+  return `${baseUrl(env)}/json/${feedId}`;
+}
+
 export function feedUrl(
   format: "rss" | "atom",
   feedId: string,
@@ -21,6 +25,34 @@ export function feedUrl(
   return format === "rss" ? feedRssUrl(feedId, env) : feedAtomUrl(feedId, env);
 }
 
+export type FeedFormat = "rss" | "atom" | "json";
+
+export function feedFormatUrl(
+  format: FeedFormat,
+  feedId: string,
+  env: Env,
+): string {
+  if (format === "atom") return feedAtomUrl(feedId, env);
+  if (format === "json") return feedJsonUrl(feedId, env);
+  return feedRssUrl(feedId, env);
+}
+
+/**
+ * Link to a third-party validator for the public feed URL: the W3C Feed
+ * Validator for RSS/Atom, validator.jsonfeed.org for JSON Feed. Used in the
+ * admin UI so an operator can confirm a feed parses in real readers.
+ */
+export function feedValidatorUrl(
+  format: FeedFormat,
+  feedId: string,
+  env: Env,
+): string {
+  const encoded = encodeURIComponent(feedFormatUrl(format, feedId, env));
+  return format === "json"
+    ? `https://validator.jsonfeed.org/?url=${encoded}`
+    : `https://validator.w3.org/feed/check.cgi?url=${encoded}`;
+}
+
 export function feedEmailAddress(mailboxId: string, env: Env): string {
   // The mailbox→address shape lives on the VO; this edge only resolves the domain.
   return MailboxId.unchecked(mailboxId).emailAddress(