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/Support/AssetStorage/AssetStorage.php

@@ -59,9 +59,16 @@ class AssetStorage
         return Storage::get($asset->disk_path);
     }
 
+    /**
+     * A URL to view/download this asset. Not Storage::url() — the default
+     * 'local' disk has no configured URL driver and throws if asked for
+     * one, so this goes through the authenticated streaming route instead.
+     */
     public function url(Asset $asset): ?string
     {
-        return Storage::exists($asset->disk_path) ? Storage::url($asset->disk_path) : null;
+        return Storage::exists($asset->disk_path)
+            ? route('video-projects.assets.show', [$asset->video_project_id, $asset->id])
+            : null;
     }
 
     public function delete(Asset $asset): void