refactor(domain): consolidate Feed aggregate invariants in domain/feed.ts

Gather the feed's scattered business rules — expiry, sender allow/block policy,
and the email byte-size budget — into one framework-agnostic module. Expiry was
duplicated across feed-service, email-processor and the rss/atom/entries routes;
the sender policy and trim loop lived inline in email-processor. Each now calls
a single function (isExpired, applySenderPolicy, trimToByteBudget,
resolveExpiresAt). Drops the now-unused MAX_METADATA_EMAILS constant.

Behaviour-preserving; adds feed.test.ts covering every invariant.

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

src/routes/atom.ts

@@ -3,6 +3,7 @@ import { Env } from "../types";
 import { generateAtomFeed } from "../utils/feed-generator";
 import { fetchFeedData } from "../utils/feed-fetcher";
 import { baseUrl, feedAtomUrl } from "../utils/urls";
+import { isExpired } from "../domain/feed";
 
 export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   try {
@@ -15,10 +16,7 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
     if (!feedData) {
       return new Response("Feed not found", { status: 404 });
     }
-    if (
-      feedData.feedConfig.expires_at !== undefined &&
-      feedData.feedConfig.expires_at <= Date.now()
-    ) {
+    if (isExpired(feedData.feedConfig)) {
       return new Response("Feed has expired", { status: 410 });
     }