Adopt Laravel's Image facade for media processing, upgrade Intervention to v4

Laravel 13's Image facade wraps Intervention Image v4 internally, so
switching our upload width probe and resize job/command to it required
bumping intervention/image ^3 -> ^4 (and its intervention/gif ^5
dependency). Removes our own ImageManager container binding and
config/image.php in favour of Laravel's built-in driver resolution.

Also fixes a latent filename mismatch in ProcessMediaJobTest that Pint's
stricter typing on the new Image API turned into a hard TypeError, and
tidies config/flare.php to use imported class names instead of FQCNs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018Sorsgn85nw7uQyRMNvzyD

composer.json

@@ -15,7 +15,7 @@
         "ext-sodium": "*",
         "cviebrock/eloquent-sluggable": "^13.0",
         "indieauth/client": "^1.1",
-        "intervention/image": "^3",
+        "intervention/image": "^4.0",
         "jonnybarnes/indieweb": "~0.2",
         "jonnybarnes/webmentions-parser": "~0.5",
         "laravel/framework": "^13.0",