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/AiProviderDriver.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Support\AiProviders;
+
+use App\Models\AiProvider;
+
+interface AiProviderDriver
+{
+    /**
+     * Generate content from a prompt. What "content" means depends on the
+     * provider — text for a text generator, image bytes for an image
+     * generator, audio bytes for a speech synthesizer. $options overrides
+     * $provider->config for this call only (e.g. model, voice_id).
+     *
+     * @param  array<string, mixed>  $options
+     *
+     * @throws AiGenerationException on any failure (bad credentials,
+     *                                unreachable API, rejected prompt, unexpected response shape).
+     */
+    public function generate(AiProvider $provider, string $prompt, array $options = []): AiGenerationResult;
+}