Add AI provider abstraction and asset storage (tasks #33, #34)

AiProviderDriver: one generic interface (generate(prompt, options) ->
AiGenerationResult) rather than per-capability interfaces, since text
(OpenAI chat completions), image (StabilityAI stable-image), and
speech (ElevenLabs TTS) generation all reduce to prompt-in/content-out.
Drivers throw AiGenerationException on failure; AiProviderManager
resolves the right driver from an AiProvider record's `provider`
column and guards on is_active. Real request/response shapes per each
provider's API, verified with Http::fake() — no live API keys
available yet to smoke test against the real services.

AssetStorage: stores generated content under
video-projects/{id}/{type}/v{n}.{ext}, versioned per (project, type[,
step]) slot so retries don't clobber earlier output, plus
contents/url/delete and pruneOldVersions for cleanup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Providers/AppServiceProvider.php

@@ -2,6 +2,10 @@
 
 namespace App\Providers;
 
+use App\Support\AiProviders\AiProviderManager;
+use App\Support\AiProviders\ElevenLabsDriver;
+use App\Support\AiProviders\OpenAiDriver;
+use App\Support\AiProviders\StabilityAiDriver;
 use App\Support\Publishing\LinkedInPublisher;
 use App\Support\Publishing\PublisherManager;
 use App\Support\Publishing\RedditPublisher;
@@ -22,6 +26,12 @@ class AppServiceProvider extends ServiceProvider
             $app->make(TwitterPublisher::class),
             $app->make(LinkedInPublisher::class),
         ]));
+
+        $this->app->singleton(AiProviderManager::class, fn ($app) => new AiProviderManager([
+            'openai' => $app->make(OpenAiDriver::class),
+            'stability_ai' => $app->make(StabilityAiDriver::class),
+            'elevenlabs' => $app->make(ElevenLabsDriver::class),
+        ]));
     }
 
     /**