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/index.ts

@@ -3,6 +3,7 @@ import { cors } from "hono/cors";
 import { handle as handleInbound } from "./routes/inbound";
 import { handle as handleRSS } from "./routes/rss";
 import { handle as handleAtom } from "./routes/atom";
+import { handle as handleJSON } from "./routes/json";
 import { handle as handleAdmin } from "./routes/admin";
 import { handle as handleEntry } from "./routes/entries";
 import { handle as handleFiles } from "./routes/files";
@@ -116,6 +117,7 @@ app.use(
 const api = new Hono<AppEnv>();
 const rss = new Hono<AppEnv>();
 const atom = new Hono<AppEnv>();
+const json = new Hono<AppEnv>();
 const entries = new Hono<AppEnv>();
 const files = new Hono<AppEnv>();
 const admin = new Hono<AppEnv>();
@@ -151,6 +153,9 @@ rss.get("/:feedId", handleRSS);
 // Atom feed routes (public)
 atom.get("/:feedId", handleAtom);
 
+// JSON Feed routes (public)
+json.get("/:feedId", handleJSON);
+
 // Email entry HTML view (public)
 entries.get("/:feedId/:entryId", handleEntry);
 
@@ -166,6 +171,7 @@ app.route("/api", api);
 app.route("/api", apiApp);
 app.route("/rss", rss);
 app.route("/atom", atom);
+app.route("/json", json);
 app.route("/entries", entries);
 app.route("/files", files);
 app.route("/admin", admin);