Add worker-aware Media app (issues #2, #3)

[acourtiol]

Companion PR — pub/get.php shim + composer.json extra.map update will land at opengento/magento2-frankenphp-base once this is merged (or in parallel if you'd like to review them together).

Problem this addresses

Two open issues on the sibling magento2-frankenphp-base repo:

• #2 Product Image Cache generation failure — PLP/PDP product images return 404 because pub/get.php isn't replaced by the worker shim, so it runs stock Magento pub/get.php in classic mode. Under high media load this is either slow (~150–250 ms bootstrap per missing-media request) or fails outright depending on how FrankenPHP routes the request.
• #3 Make all entry points benefit of shared bootstrap — App\Http and App\StaticResource go through BootstrapPool and reuse the hot ObjectManager. Media doesn't. This PR closes that gap for Magento\MediaStorage\App\Media.

Approach

New class: Opengento\Application\App\Media — sibling to App\Http and App\StaticResource. It's the third worker-mode entry point.

Per request, it:

1. Snapshots State::$_areaCode (via reflection — see § Why reflection)
2. Sets the field to null
3. Constructs Magento\MediaStorage\App\Media via the worker's shared ObjectManager with request-derived (mediaDirectory, configCacheFile, isAllowed, relativeFileName) arguments — same contract that stock pub/get.php builds
4. Delegates to MagentoMedia::launch() (which sets area to GLOBAL internally, materializes the file, returns the FileResponse)
5. In finally{}, restores the original area code so the next /index.php or /static.php request on the same worker isn't affected

The companion PR on magento2-frankenphp-base ships:

• a new pub/get.php that does \$frankengento = \Opengento\Application\App\Media::class; include 'worker.php';
• a corresponding extra.map entry so the file lands in the project's pub/ directory at install time

Why reflection<a id="why-reflection">

`MagentoMedia::launch()` unconditionally calls `State::setAreaCode(Area::AREA_GLOBAL)` on every invocation. In worker mode the `State` singleton persists across requests and `State::setAreaCode()` throws "Area code is already set" on the second call.

`State` doesn't implement `ResetAfterRequestInterface` and isn't auto-tracked by `AppObjectManager`'s `Resetter` (it's resolved via `get()` not `create()`), so the resetter never clears `_areaCode` between requests.

`State` also exposes no public reset API:

• `setAreaCode` throws
• `emulateAreaCode` wraps a callback and restores, but Magento Media itself calls `setAreaCode` inside the callback — which still throws
• `getAreaCode` throws when unset (so we can't even read the field cleanly to snapshot it)

Reflection on `_areaCode` is the smallest contained workaround. Save+restore lives in a `finally{}` so a media-app failure can't leave the worker in a broken area state for other endpoints.

If/when Magento adds `ResetAfterRequestInterface` to `State` (or exposes a public `clear()`), the reflection here can be deleted.

Open questions / design notes

1. Should `App\Media` extend or wrap `MagentoMedia` rather than instantiate-and-delegate via `ObjectManager->create()`? I went with delegation to keep our class small and avoid coupling to `MagentoMedia` constructor params changing.
2. Is there a cleaner alternative to reflection here? I couldn't find one — open to suggestions in review.

Stacking

Standalone — doesn't depend on any in-flight PR on this repo. The companion PR at `magento2-frankenphp-base` depends on this being merged (the new `pub/get.php` shim references `Opengento\Application\App\Media`).