Add thumbnail, metadata, and YouTube publishing handlers (#40, #41, #42)

ThumbnailGenerationHandler (#40): generates thumbnail candidates via
StabilityAI, one per style (default: two contrasting styles) — all
stored as assets on the same step, so the existing approval UI's
"show every asset on this step side by side" behavior is the A/B
comparison the task calls for, no extra UI needed.

MetadataGenerationHandler (#41): SEO title/description/tags/hashtags
from the project's script via OpenAI in JSON mode, same pattern as
StoryboardHandler.

YoutubePublishHandler (#42): real OAuth2 + YouTube Data API v3
resumable upload (token refresh -> init session -> PUT video bytes),
supporting scheduling via publishAt (forces privacyStatus=private per
YouTube's own constraint). Reuses the AiProvider table for credentials
(provider='youtube', api_key=refresh_token, config=client_id/secret)
rather than a new table — added 'youtube' to AiProvider::PROVIDERS.
Does not cover analytics retrieval: that's an ongoing query against a
published video, not a one-shot publish action, so it doesn't fit the
per-video step model at all.

All 8 WorkflowStep::TYPES now have real handlers, so the "unhandled
type" test fixtures (which used real-but-not-yet-implemented types)
would go stale every time a new module lands — switched them to
fabricated type strings so they stay meaningful regardless of what
gets built next.

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

tests/Feature/Livewire/VideoProjectsShowTest.php

@@ -31,9 +31,11 @@ test('starting the default pipeline creates steps and queues the first handled o
 
 test('starting from a template uses its steps and does not queue unhandled types', function () {
     Queue::fake();
+    // Made-up types — not real WorkflowStep::TYPES values — so this test
+    // stays valid regardless of which real types have handlers over time.
     $template = WorkflowTemplate::factory()->create(['steps' => [
-        ['type' => 'thumbnail_generation'],
-        ['type' => 'youtube_publish'],
+        ['type' => 'some_future_step_type'],
+        ['type' => 'another_future_step_type'],
     ]]);
     $project = VideoProject::factory()->create();
 
@@ -44,7 +46,6 @@ test('starting from a template uses its steps and does not queue unhandled types
     expect($project->fresh()->workflow_template_id)->toBe($template->id);
     expect($project->steps()->count())->toBe(2);
 
-    // thumbnail_generation has no handler yet, so nothing should be queued
     Queue::assertNotPushed(RunWorkflowStepJob::class);
 });