Add storyboard/video-gen/video-editing handlers and asset previews (#37-39, #45, #46)

StoryboardHandler (#37): breaks the project's script into a numbered
scene-by-scene breakdown (description + image prompt per scene) via
OpenAI in JSON mode (added json_mode support to OpenAiDriver for
reliable structured output), stored as a JSON asset.

VideoGenerationHandler (#38): generates one still image per storyboard
scene via StabilityAI — the scene *visuals*, not motion/video clips
(no text-to-video model integrated; see the task comment for why).

VideoEditingHandler (#39): the step that actually produces a video
file — assembles the scene stills into an MP4 via ffmpeg's concat
demuxer, timed to the voiceover audio's duration (probed via ffprobe)
or a fixed per-scene duration if there's no audio, then muxes the
audio track in. Verified against the real ffmpeg/ffprobe binaries on
this box, not just Process::fake().

Asset previews (#45, #46): a reusable partial renders each asset
inline by type — image/audio/video get real playback elements, text/
JSON gets a truncated <pre> block, everything else a download link.
Wired into both the awaiting_approval step view (so approving/
rejecting is an actual content review, not just a word count) and a
general project assets grid.

Also fixes a latent bug in AssetStorage::url(), found while wiring up
previews: it called Storage::url() on the default 'local' disk, which
has no configured URL driver and throws in production — only worked
before because Storage::fake() is more permissive than the real local
driver. Now serves assets through a new authenticated streaming route
instead (VideoProjectAssetController).

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

app/Http/Controllers/VideoProjectAssetController.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace App\Http\Controllers;
+
+use App\Models\Asset;
+use App\Models\VideoProject;
+use Illuminate\Support\Facades\Storage;
+use Symfony\Component\HttpFoundation\StreamedResponse;
+
+class VideoProjectAssetController extends Controller
+{
+    public function show(VideoProject $videoProject, Asset $asset): StreamedResponse
+    {
+        abort_if($asset->video_project_id !== $videoProject->id, 404);
+        abort_unless(Storage::exists($asset->disk_path), 404);
+
+        return Storage::response($asset->disk_path, null, [
+            'Content-Type' => $asset->mime_type ?: 'application/octet-stream',
+        ]);
+    }
+}