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.

templates/layout.php

@@ -59,11 +59,14 @@ $total = (int) $page['total'];
             <span class="brand-mark" aria-hidden="true"></span>
             <span class="brand-text">RSS <em>Feed</em> Dashboard</span>
         </a>
-        <div class="header-meta d-none d-md-flex align-items-center">
-            <span class="pulse" aria-hidden="true"></span>
-            <span class="mono"><?= $total ?> stories</span>
-            <span class="sep">/</span>
-            <span class="mono">YouTube · Reddit · RT</span>
+        <div class="d-flex align-items-center gap-3">
+            <div class="header-meta d-none d-md-flex align-items-center">
+                <span class="pulse" aria-hidden="true"></span>
+                <span class="mono"><?= $total ?> stories</span>
+                <span class="sep">/</span>
+                <span class="mono">YouTube · Reddit · RT</span>
+            </div>
+            <a class="manage-link" href="manage.php">Manage</a>
         </div>
     </div>
     <div class="spectrum-rail" aria-hidden="true"></div>