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/atom.test.ts

@@ -144,4 +144,117 @@ describe("Atom Feed Route", () => {
       expect(body).toContain('xmlns="http://www.w3.org/2005/Atom"');
     });
   });
+
+  describe("conditional GET (ETag + Last-Modified)", () => {
+    const FEED_ID = "test-feed-atom-cget";
+    const EMAIL_RECEIVED_AT = 1700000001000;
+
+    beforeEach(async () => {
+      const emailKey = `feed:${FEED_ID}:${EMAIL_RECEIVED_AT}`;
+      await mockEnv.EMAIL_STORAGE.put(
+        emailKey,
+        JSON.stringify({
+          subject: "Atom Subject",
+          from: "Sender <sender@example.com>",
+          content: "<p>Body</p>",
+          receivedAt: EMAIL_RECEIVED_AT,
+          headers: {},
+        }),
+      );
+      await mockEnv.EMAIL_STORAGE.put(
+        `feed:${FEED_ID}:metadata`,
+        JSON.stringify({
+          emails: [
+            {
+              key: emailKey,
+              subject: "Atom Subject",
+              receivedAt: EMAIL_RECEIVED_AT,
+            },
+          ],
+        }),
+      );
+      await mockEnv.EMAIL_STORAGE.put(
+        `feed:${FEED_ID}:config`,
+        JSON.stringify({
+          title: "Atom Cget Feed",
+          language: "en",
+          created_at: 1700000000000,
+        }),
+      );
+    });
+
+    it("first GET returns 200 with ETag and Last-Modified headers", async () => {
+      const res = await testApp.request(`/${FEED_ID}`, {}, mockEnv);
+      expect(res.status).toBe(200);
+      expect(res.headers.get("ETag")).toBeTruthy();
+      expect(res.headers.get("Last-Modified")).toBeTruthy();
+    });
+
+    it("GET with matching If-None-Match returns 304 with empty body", async () => {
+      const first = await testApp.request(`/${FEED_ID}`, {}, mockEnv);
+      const etag = first.headers.get("ETag")!;
+
+      const res = await testApp.request(
+        `/${FEED_ID}`,
+        { headers: { "If-None-Match": etag } },
+        mockEnv,
+      );
+      expect(res.status).toBe(304);
+      expect(await res.text()).toBe("");
+    });
+
+    it("GET with If-Modified-Since in the future returns 304", async () => {
+      const future = new Date(EMAIL_RECEIVED_AT + 1000).toUTCString();
+      const res = await testApp.request(
+        `/${FEED_ID}`,
+        { headers: { "If-Modified-Since": future } },
+        mockEnv,
+      );
+      expect(res.status).toBe(304);
+    });
+
+    it("stale If-None-Match after new email results in 200", async () => {
+      const first = await testApp.request(`/${FEED_ID}`, {}, mockEnv);
+      const oldEtag = first.headers.get("ETag")!;
+
+      const newReceivedAt = EMAIL_RECEIVED_AT + 5000;
+      const newEmailKey = `feed:${FEED_ID}:${newReceivedAt}`;
+      await mockEnv.EMAIL_STORAGE.put(
+        newEmailKey,
+        JSON.stringify({
+          subject: "Newer Atom Email",
+          from: "Sender <sender@example.com>",
+          content: "<p>New body</p>",
+          receivedAt: newReceivedAt,
+          headers: {},
+        }),
+      );
+      await mockEnv.EMAIL_STORAGE.put(
+        `feed:${FEED_ID}:metadata`,
+        JSON.stringify({
+          emails: [
+            {
+              key: newEmailKey,
+              subject: "Newer Atom Email",
+              receivedAt: newReceivedAt,
+            },
+            {
+              key: `feed:${FEED_ID}:${EMAIL_RECEIVED_AT}`,
+              subject: "Atom Subject",
+              receivedAt: EMAIL_RECEIVED_AT,
+            },
+          ],
+        }),
+      );
+
+      const res = await testApp.request(
+        `/${FEED_ID}`,
+        { headers: { "If-None-Match": oldEtag } },
+        mockEnv,
+      );
+      expect(res.status).toBe(200);
+      const newEtag = res.headers.get("ETag");
+      expect(newEtag).not.toBe(oldEtag);
+    });
+  });
 });