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/Support/AiProviders/AiProviderManager.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace App\Support\AiProviders;
+
+use App\Models\AiProvider;
+
+class AiProviderManager
+{
+    /**
+     * @param  array<string, AiProviderDriver>  $drivers  Keyed by AiProvider::PROVIDERS value.
+     */
+    public function __construct(private readonly array $drivers) {}
+
+    public function driverFor(AiProvider $provider): AiProviderDriver
+    {
+        if (! isset($this->drivers[$provider->provider])) {
+            throw new AiGenerationException("No driver registered for AI provider '{$provider->provider}'.");
+        }
+
+        return $this->drivers[$provider->provider];
+    }
+
+    /**
+     * @param  array<string, mixed>  $options
+     */
+    public function generate(AiProvider $provider, string $prompt, array $options = []): AiGenerationResult
+    {
+        if (! $provider->is_active) {
+            throw new AiGenerationException("AI provider '{$provider->name}' is not active.");
+        }
+
+        return $this->driverFor($provider)->generate($provider, $prompt, $options);
+    }
+}