feat: reader-compat batch — JSON Feed, OPML export, conditional GET, dedup

Batch of four reader-facing improvements (TODO "Compat lecteurs + dedup"):

- JSON Feed at /json/:feedId (feed lib .json1()); all formats cross-link
- OPML export at /admin/opml (admin-protected; the registry lists every
  feed URL, so it must not be public)
- Conditional GET on /rss + /atom: strong ETag + Last-Modified, 304 on
  If-None-Match/If-Modified-Since, validators shared via http-cache.ts
- Duplicate-send dedup in ingestion: match by Message-ID, fall back to a
  SHA-256 of normalized subject+content; a duplicate is a no-op and bumps
  the new emails_deduplicated counter (status page + /api/v1/stats)

429 tests green, tsc clean, build dry-run OK. Docs (README/CLAUDE/TODO +
landing cards) updated.

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

src/infrastructure/feed-generator.ts

@@ -30,7 +30,7 @@ function buildFeed(
   emails: EmailData[],
   baseUrl: string,
   feedId: string,
-  selfUrl?: { rss?: string; atom?: string },
+  selfUrl?: { rss?: string; atom?: string; json?: string },
 ): Feed {
   const iconUrl = `${baseUrl}/favicon/${feedId}`;
   const feed = new Feed({
@@ -52,6 +52,7 @@ function buildFeed(
     feedLinks: {
       rss: selfUrl?.rss ?? `${baseUrl}/rss/${feedId}`,
       atom: selfUrl?.atom ?? `${baseUrl}/atom/${feedId}`,
+      json: selfUrl?.json ?? `${baseUrl}/json/${feedId}`,
     },
     author: feedConfig.author
       ? {
@@ -127,3 +128,19 @@ export function generateAtomFeed(
     ).atom1(),
   );
 }
+
+export function generateJsonFeed(
+  feedConfig: FeedConfig,
+  emails: EmailData[],
+  baseUrl: string,
+  feedId: string,
+  selfUrl?: string,
+): string {
+  return buildFeed(
+    feedConfig,
+    emails,
+    baseUrl,
+    feedId,
+    selfUrl ? { json: selfUrl } : undefined,
+  ).json1();
+}