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/rss.ts

@@ -5,6 +5,11 @@ import { fetchFeedData } from "../application/feed-fetcher";
 import { baseUrl, feedRssUrl } from "../infrastructure/urls";
 import { isExpired } from "../domain/feed";
 import { FeedId } from "../domain/value-objects/feed-id";
+import {
+  computeFeedValidators,
+  isNotModified,
+  notModifiedResponse,
+} from "../infrastructure/http-cache";
 
 export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   try {
@@ -21,6 +26,17 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
       return new Response("Feed has expired", { status: 410 });
     }
 
+    const validators = computeFeedValidators(
+      "rss",
+      feedId,
+      feedData.feedConfig,
+      feedData.emails,
+    );
+
+    if (isNotModified(c.req.raw, validators)) {
+      return notModifiedResponse(validators);
+    }
+
     const base = baseUrl(c.env);
     const selfUrl = new URL(c.req.url).origin + `/rss/${feedId}`;
     const rssXml = generateRssFeed(
@@ -42,6 +58,8 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
         "Cache-Control": "max-age=1800",
         "X-Robots-Tag": "noindex",
         Link: linkHeader,
+        ETag: validators.etag,
+        "Last-Modified": validators.lastModified,
       },
     });
   } catch (error) {