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/Providers/AppServiceProvider.php

@@ -13,7 +13,10 @@ use App\Support\Publishing\TwitterPublisher;
 use App\Support\Publishing\WordPressPublisher;
 use App\Support\VideoWorkflow\StepHandlers\ScriptGenerationHandler;
 use App\Support\VideoWorkflow\StepHandlers\StepHandlerManager;
+use App\Support\VideoWorkflow\StepHandlers\StoryboardHandler;
 use App\Support\VideoWorkflow\StepHandlers\TtsVoiceoverHandler;
+use App\Support\VideoWorkflow\StepHandlers\VideoEditingHandler;
+use App\Support\VideoWorkflow\StepHandlers\VideoGenerationHandler;
 use Illuminate\Support\ServiceProvider;
 
 class AppServiceProvider extends ServiceProvider
@@ -39,6 +42,9 @@ class AppServiceProvider extends ServiceProvider
         $this->app->singleton(StepHandlerManager::class, fn ($app) => new StepHandlerManager([
             $app->make(ScriptGenerationHandler::class),
             $app->make(TtsVoiceoverHandler::class),
+            $app->make(StoryboardHandler::class),
+            $app->make(VideoGenerationHandler::class),
+            $app->make(VideoEditingHandler::class),
         ]));
     }