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/routes/json.ts

@@ -0,0 +1,51 @@
+import { Context } from "hono";
+import { Env } from "../types";
+import { generateJsonFeed } from "../infrastructure/feed-generator";
+import { fetchFeedData } from "../application/feed-fetcher";
+import { baseUrl } from "../infrastructure/urls";
+import { isExpired } from "../domain/feed";
+import { FeedId } from "../domain/value-objects/feed-id";
+
+export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
+  try {
+    const feedId = c.req.param("feedId");
+    if (!feedId) {
+      return new Response("Feed ID is required", { status: 400 });
+    }
+
+    const feedData = await fetchFeedData(FeedId.unchecked(feedId), c.env);
+    if (!feedData) {
+      return new Response("Feed not found", { status: 404 });
+    }
+    if (isExpired(feedData.feedConfig)) {
+      return new Response("Feed has expired", { status: 410 });
+    }
+
+    const base = baseUrl(c.env);
+    const selfUrl = new URL(c.req.url).origin + `/json/${feedId}`;
+    const jsonFeed = generateJsonFeed(
+      feedData.feedConfig,
+      feedData.emails,
+      base,
+      feedId,
+      selfUrl,
+    );
+    const linkHeader = [
+      `<${base}/hub>; rel="hub"`,
+      `<${selfUrl}>; rel="self"`,
+    ].join(", ");
+
+    return new Response(jsonFeed, {
+      status: 200,
+      headers: {
+        "Content-Type": "application/feed+json",
+        "Cache-Control": "max-age=1800",
+        "X-Robots-Tag": "noindex",
+        Link: linkHeader,
+      },
+    });
+  } catch (error) {
+    console.error("Error generating JSON feed:", error);
+    return new Response("Internal Server Error", { status: 500 });
+  }
+}