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.

config/app.php

@@ -35,4 +35,9 @@ return [
 
     // Identify the client to feed providers (important for Reddit).
     'user_agent' => 'RSSFeed/1.0 (+https://localhost; contact: admin@localhost)',
+
+    // Password for manage.php (manual subreddit/channel configuration).
+    // Accepts a plain password or a password_hash() value; empty disables the
+    // manage page entirely. Change this before exposing the app publicly.
+    'manage_password' => 'rssfeed-admin',
 ];