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/entries.ts

@@ -4,6 +4,7 @@ import { Env } from "../types";
 import { processEmailContent } from "../utils/html-processor";
 import { formatBytes } from "../utils/format";
 import { FeedRepository } from "../domain/feed-repository";
+import { isExpired } from "../domain/feed";
 
 export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   const feedId = c.req.param("feedId");
@@ -22,10 +23,7 @@ export async function handle(c: Context<{ Bindings: Env }>): Promise<Response> {
   if (!feedMetadata) {
     return new Response("Feed not found", { status: 404 });
   }
-  if (
-    feedConfig?.expires_at !== undefined &&
-    feedConfig.expires_at <= Date.now()
-  ) {
+  if (feedConfig && isExpired(feedConfig)) {
     return new Response("Feed has expired", { status: 410 });
   }