feat(feeds): add configurable per-feed lifetime (TTL)

Replace the demo nightly KV wipe with a per-feed expiry. Feeds can be
given a lifetime at creation (and edited later); FEED_TTL_HOURS locks the
value server-side and greys out the UI field. Expired feeds stay visible
in admin (greyed, actions disabled), return 410 on rss/atom/entries, and
reject inbound emails. The scheduled handler now purges only expired
feeds (KV + R2 attachments) on an hourly global cron.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/styles/components.css

@@ -972,3 +972,41 @@ table.table code {
   padding: 0;
   margin: 0;
 }
+
+/* Feed TTL — expiry badges */
+.pill-expiry {
+  background-color: rgba(255, 159, 10, 0.15);
+  color: var(--color-warning);
+  border-color: rgba(255, 159, 10, 0.3);
+}
+
+.pill-expired {
+  background-color: rgba(255, 69, 58, 0.15);
+  color: var(--color-danger);
+  border-color: rgba(255, 69, 58, 0.3);
+}
+
+/* Feed TTL — expired feed row */
+.feed-expired .feed-header,
+.feed-expired td:not(:last-child) {
+  opacity: 0.5;
+}
+
+/* Feed TTL — disabled-looking action buttons on expired feeds */
+.button-disabled {
+  opacity: 0.35;
+  cursor: not-allowed;
+  pointer-events: none;
+  user-select: none;
+}
+
+/* Feed TTL — card states in edit form */
+.card-warning {
+  border-color: rgba(255, 159, 10, 0.4);
+  background-color: rgba(255, 159, 10, 0.06);
+}
+
+.card-disabled {
+  opacity: 0.6;
+  pointer-events: none;
+}