Add manual subreddit/YouTube channel configuration (manage.php)

Browser-based feed selection layered on top of config/feeds.php:

- manage.php (password + CSRF gated, session based): add subreddits
  (name / r/name / URL) and YouTube channels (@handle / UC id / URL,
  handle resolved to channel_id once, youtube.com only); enable/disable
  toggles; remove manual feeds; config feeds are hidden, not deleted.
- New feeds are validated with one immediate fetch (bypassing the
  per-refresh host cap): HTTP 404 rejects the entry, 429 keeps it for
  a later refresh. YouTube labels adopt the real channel title.
- feeds table gains origin (config|manual) and enabled columns, with
  a guarded ALTER migration for existing databases.
- Only enabled feeds are refreshed and served: getItems/countItems
  filter by feed_url in enabled feeds, so deselected feeds' cached
  stories are hidden, not deleted; re-adding a URL re-enables it.
- Dashboard header gets a Manage link; manage page styled with the
  existing design tokens; README documents the feature.

assets/manage.js

@@ -0,0 +1,11 @@
+(function () {
+    'use strict';
+
+    document.querySelectorAll('form[data-confirm]').forEach(function (form) {
+        form.addEventListener('submit', function (event) {
+            if (!window.confirm(form.dataset.confirm)) {
+                event.preventDefault();
+            }
+        });
+    });
+})();