Phase 14e-2: domain-event listeners (file cleanup + cache invalidation)

[bigin]

Summary

Scriptor-side companion to 14e-1 (iManager PSR-14 dispatcher + storage
emits). The container now wires a sync dispatcher on top of the
in-memory subscriber provider, hands it to SqliteStorage, and
subscribes two listeners.

Scriptor\Boot\Events\ItemFileCleanupListener (subscribes to
ItemDeleted):

• Walks FileRepository::findByItem() while the row is still
  reachable (Plan §14e contract: dispatcher fires before the SQL
  DELETE so the FK cascade hasn't dropped the metadata yet) and asks
  FileStorage to drop each asset.
• Also scrubs every <W>x<H>_<file> thumbnail under <dir>/thumbnail/
  via scandir, matching the upload convention. Tolerates missing
  dirs and unreadable siblings.

Scriptor\Boot\Events\PageCacheInvalidationListener (subscribes
to ItemCreated / ItemUpdated / ItemDeleted):

• Filters by category id (Pages only — Users mutations don't
  invalidate the rendered-HTML cache).
• Calls CacheInterface::clear(). BasicTheme keys cache entries by
  md5(host + REQUEST_URI), which we can't recreate at event-time,
  so this is a global flush until the cache learns tags / per-page
  prefixes.
  prefixes.

ImanagerBootstrap:

• Registers SubscriberListenerProvider + alias to
  ListenerProviderInterface + EventDispatcherInterface
  (SyncEventDispatcher composing the provider).
• SqliteStorage is now built with the dispatcher.
• wireDomainEventListeners() subscribes both listeners through the
  provider; listener instances are constructed lazily on first fire,
  and the cache-invalidator caches its watched-category id after the
  first lookup.

composer.lock: pulls in psr/event-dispatcher 1.0.0 (transitively
through bigins/imanager at 14e-1).

Test plan

• Create page → 302 (ItemCreated dispatched)
• Upload image → 200; fileId, asset + 300×300 thumb on disk
• Hit frontend → 1 cache file
• Delete page → 302
• After delete:
  • items row gone (FK cascade)
  • files row gone (FK cascade)
  • asset 14e2.png gone (ItemFileCleanupListener)
  • thumbnail/300x300_14e2.png gone (ItemFileCleanupListener)
  • cache files: 0 (PageCacheInvalidationListener)
• Browser smoke against ServBay