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.

src/FeedService.php

@@ -39,8 +39,9 @@ final class FeedService
         ];
 
         $hostCounts = [];
+        $activeFeeds = $this->repository->enabledFeeds();
 
-        foreach ($this->repository->staleFeeds($this->feeds, $this->ttlSeconds, $this->errorRetryTtlSeconds) as $feed) {
+        foreach ($this->repository->staleFeeds($activeFeeds, $this->ttlSeconds, $this->errorRetryTtlSeconds) as $feed) {
             $url = (string) $feed['url'];
             $host = (string) parse_url($url, PHP_URL_HOST);
 
@@ -100,12 +101,14 @@ final class FeedService
     }
 
     /**
+     * Sources that currently have at least one enabled feed.
+     *
      * @return array<int, string>
      */
     public function sources(): array
     {
         $sources = [];
-        foreach ($this->feeds as $feed) {
+        foreach ($this->repository->enabledFeeds() as $feed) {
             $sources[(string) $feed['source']] = true;
         }