Add configurable visit storage setting to disable DB tracking

Problem

Some sites use external analytics exclusively (Matomo, Piano, GA) and do not need the built-in shorturl_visits database tracking. Currently, the VisitDatabaseSubscriber always writes to the DB, and the stats tab/API always display. There is no way to disable the DB storage without uninstalling or hacking code.

Proposed solution

Add two hierarchical settings to shorturl.settings:

Settings form UI

[x] Enable visit tracking
    Tracks visits to short URL redirects. Dispatches a visit
    event that analytics subscribers can consume.

    [x] Enable built-in statistics
        Stores visit data in the database for the statistics
        dashboard and API endpoints.

• Enable visit tracking (checkbox, default: on)
  • Unchecked: middleware and all subscribers removed from container. Nothing tracked. Zero overhead.
  • Checked: middleware active, ShortUrlVisitEvent dispatched on every redirect. External analytics subscribers receive events.
• Enable built-in statistics (checkbox, default: on, visible only when tracking is enabled)
  • Checked: VisitDatabaseSubscriber active, stats tab and API visible, purge cron active.
  • Unchecked: DB subscriber removed, stats tab hidden, API returns 403, purge skipped. Event still dispatches for external analytics.

Config keys

# shorturl.settings
enable_tracking: true
enable_builtin_stats: true

Implementation

Use the ServiceProvider::alter() pattern to remove services at compile time based on config. This means zero runtime overhead — disabled services are never instantiated.

// In ShorturlServiceProvider::alter():
$config = BootstrapConfigStorageFactory::get()
    ->read('shorturl.settings');

if (empty($config['enable_tracking'])) {
    // No tracking at all.
    $container->removeDefinition(
        'Drupal\shorturl\EventSubscriber\VisitDatabaseSubscriber'
    );
    $container->removeDefinition(
        'Drupal\shorturl\StackMiddleware\ShortUrlVisitMiddleware'
    );
}
elseif (empty($config['enable_builtin_stats'])) {
    // Event dispatches for external analytics,
    // but no DB writes.
    $container->removeDefinition(
        'Drupal\shorturl\EventSubscriber\VisitDatabaseSubscriber'
    );
}

Affected areas

Depends on #3584485 (visit event system).