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/Database.php

@@ -58,8 +58,26 @@ final class Database
                 label           TEXT,
                 last_attempt_at TEXT,
                 last_success_at TEXT,
-                last_error      TEXT
+                last_error      TEXT,
+                origin          TEXT NOT NULL DEFAULT 'config',
+                enabled         INTEGER NOT NULL DEFAULT 1
             )
             SQL);
+
+        // SQLite has no "ADD COLUMN IF NOT EXISTS": migrate pre-existing tables.
+        self::addColumn($pdo, 'feeds', 'origin', "ALTER TABLE feeds ADD COLUMN origin TEXT NOT NULL DEFAULT 'config'");
+        self::addColumn($pdo, 'feeds', 'enabled', 'ALTER TABLE feeds ADD COLUMN enabled INTEGER NOT NULL DEFAULT 1');
+    }
+
+    private static function addColumn(PDO $pdo, string $table, string $column, string $sql): void
+    {
+        $existing = [];
+        foreach ($pdo->query(sprintf('PRAGMA table_info(%s)', $table)) as $row) {
+            $existing[strtolower((string) $row['name'])] = true;
+        }
+
+        if (!isset($existing[strtolower($column)])) {
+            $pdo->exec($sql);
+        }
     }
 }