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/AiProviders/OpenAiDriver.php

@@ -16,14 +16,23 @@ class OpenAiDriver implements AiProviderDriver
     {
         $model = $options['model'] ?? $provider->config['model'] ?? 'gpt-4o-mini';
 
+        $body = [
+            'model' => $model,
+            'messages' => [
+                ['role' => 'user', 'content' => $prompt],
+            ],
+        ];
+
+        // Ask the API to guarantee valid JSON back, for callers that need
+        // structured output (e.g. a storyboard's scene breakdown). The
+        // prompt itself must still ask for JSON — this only enforces it.
+        if ($options['json_mode'] ?? false) {
+            $body['response_format'] = ['type' => 'json_object'];
+        }
+
         try {
             $response = Http::withToken($provider->api_key)
-                ->post('https://api.openai.com/v1/chat/completions', [
-                    'model' => $model,
-                    'messages' => [
-                        ['role' => 'user', 'content' => $prompt],
-                    ],
-                ]);
+                ->post('https://api.openai.com/v1/chat/completions', $body);
         } catch (ConnectionException $e) {
             throw new AiGenerationException("could not connect to OpenAI: {$e->getMessage()}");
         }